diff --git a/app/models/__pycache__/video_models.cpython-312.pyc b/app/models/__pycache__/video_models.cpython-312.pyc
new file mode 100644
index 0000000..5792e1c
Binary files /dev/null and b/app/models/__pycache__/video_models.cpython-312.pyc differ
diff --git a/app/models/video_models.py b/app/models/video_models.py
new file mode 100644
index 0000000..b6f206d
--- /dev/null
+++ b/app/models/video_models.py
@@ -0,0 +1,38 @@
+from pydantic import BaseModel
+
+
+class ExtractFramesRequest(BaseModel):
+    file_path: str
+    source_id: int
+    job_id: int
+    mode: str = "interval"
+    frame_interval: int = 30
+
+
+class VideoToTextRequest(BaseModel):
+    file_path: str
+    source_id: int
+    job_id: int
+    start_sec: float
+    end_sec: float
+    model: str | None = None
+    prompt_template: str | None = None
+
+
+class FrameInfo(BaseModel):
+    frame_index: int
+    time_sec: float
+    frame_path: str
+
+
+class VideoJobCallback(BaseModel):
+    job_id: int
+    status: str
+    frames: list[FrameInfo] | None = None
+    output_path: str | None = None
+    error_message: str | None = None
+
+
+class VideoAcceptedResponse(BaseModel):
+    message: str
+    job_id: int
diff --git a/app/routers/__pycache__/video.cpython-312.pyc b/app/routers/__pycache__/video.cpython-312.pyc
index 4566483..c3a3f06 100644
Binary files a/app/routers/__pycache__/video.cpython-312.pyc and b/app/routers/__pycache__/video.cpython-312.pyc differ
diff --git a/app/routers/video.py b/app/routers/video.py
index 136e997..efdba25 100644
--- a/app/routers/video.py
+++ b/app/routers/video.py
@@ -1,3 +1,69 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, BackgroundTasks, Depends
+
+from app.clients.llm.base import LLMClient
+from app.clients.storage.base import StorageClient
+from app.core.config import get_config
+from app.core.dependencies import get_llm_client, get_storage_client
+from app.core.exceptions import VideoTooLargeError
+from app.models.video_models import (
+    ExtractFramesRequest,
+    VideoAcceptedResponse,
+    VideoToTextRequest,
+)
+from app.services import video_service
 
 router = APIRouter(tags=["Video"])
+
+
+async def _check_video_size(storage: StorageClient, bucket: str, file_path: str, max_mb: int) -> None:
+    size_bytes = await storage.get_object_size(bucket, file_path)
+    if size_bytes > max_mb * 1024 * 1024:
+        raise VideoTooLargeError(
+            f"视频文件大小超出限制（最大 {max_mb}MB，当前 {size_bytes // 1024 // 1024}MB）"
+        )
+
+
+@router.post("/video/extract-frames", response_model=VideoAcceptedResponse, status_code=202)
+async def extract_frames(
+    req: ExtractFramesRequest,
+    background_tasks: BackgroundTasks,
+    storage: StorageClient = Depends(get_storage_client),
+) -> VideoAcceptedResponse:
+    cfg = get_config()
+    bucket = cfg["storage"]["buckets"]["source_data"]
+    max_mb = cfg["video"]["max_file_size_mb"]
+    callback_url = cfg.get("backend", {}).get("callback_url", "")
+
+    await _check_video_size(storage, bucket, req.file_path, max_mb)
+
+    background_tasks.add_task(
+        video_service.extract_frames_task,
+        req,
+        storage,
+        callback_url,
+    )
+    return VideoAcceptedResponse(message="任务已接受，后台处理中", job_id=req.job_id)
+
+
+@router.post("/video/to-text", response_model=VideoAcceptedResponse, status_code=202)
+async def video_to_text(
+    req: VideoToTextRequest,
+    background_tasks: BackgroundTasks,
+    storage: StorageClient = Depends(get_storage_client),
+    llm: LLMClient = Depends(get_llm_client),
+) -> VideoAcceptedResponse:
+    cfg = get_config()
+    bucket = cfg["storage"]["buckets"]["source_data"]
+    max_mb = cfg["video"]["max_file_size_mb"]
+    callback_url = cfg.get("backend", {}).get("callback_url", "")
+
+    await _check_video_size(storage, bucket, req.file_path, max_mb)
+
+    background_tasks.add_task(
+        video_service.video_to_text_task,
+        req,
+        llm,
+        storage,
+        callback_url,
+    )
+    return VideoAcceptedResponse(message="任务已接受，后台处理中", job_id=req.job_id)
diff --git a/app/services/__pycache__/video_service.cpython-312.pyc b/app/services/__pycache__/video_service.cpython-312.pyc
new file mode 100644
index 0000000..d6ef12d
Binary files /dev/null and b/app/services/__pycache__/video_service.cpython-312.pyc differ
diff --git a/app/services/video_service.py b/app/services/video_service.py
new file mode 100644
index 0000000..1851c0f
--- /dev/null
+++ b/app/services/video_service.py
@@ -0,0 +1,189 @@
+import base64
+import io
+import os
+import tempfile
+import time
+from typing import Callable
+
+import cv2
+import httpx
+import numpy as np
+
+from app.clients.llm.base import LLMClient
+from app.clients.storage.base import StorageClient
+from app.core.config import get_config
+from app.core.logging import get_logger
+from app.models.video_models import ExtractFramesRequest, FrameInfo, VideoToTextRequest
+
+logger = get_logger(__name__)
+
+
+async def _post_callback(url: str, payload: dict) -> None:
+    async with httpx.AsyncClient(timeout=10) as http:
+        try:
+            await http.post(url, json=payload)
+        except Exception as exc:
+            logger.error("callback_failed", extra={"url": url, "error": str(exc)})
+
+
+async def extract_frames_task(
+    req: ExtractFramesRequest,
+    storage: StorageClient,
+    callback_url: str,
+) -> None:
+    cfg = get_config()
+    bucket = cfg["storage"]["buckets"]["source_data"]
+    threshold = cfg["video"].get("keyframe_diff_threshold", 30.0)
+
+    tmp = None
+    try:
+        video_bytes = await storage.download_bytes(bucket, req.file_path)
+
+        with tempfile.NamedTemporaryFile(suffix=".mp4", delete=False) as f:
+            f.write(video_bytes)
+            tmp = f.name
+
+        cap = cv2.VideoCapture(tmp)
+        fps = cap.get(cv2.CAP_PROP_FPS) or 25.0
+        frames_info: list[FrameInfo] = []
+        upload_index = 0
+        prev_gray = None
+        frame_idx = 0
+
+        while True:
+            ret, frame = cap.read()
+            if not ret:
+                break
+
+            extract = False
+            if req.mode == "interval":
+                extract = (frame_idx % req.frame_interval == 0)
+            else:  # keyframe
+                gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY).astype(np.float32)
+                if prev_gray is None:
+                    extract = True
+                else:
+                    diff = np.mean(np.abs(gray - prev_gray))
+                    extract = diff > threshold
+                prev_gray = gray
+
+            if extract:
+                time_sec = round(frame_idx / fps, 3)
+                _, buf = cv2.imencode(".jpg", frame)
+                frame_path = f"frames/{req.source_id}/{upload_index}.jpg"
+                await storage.upload_bytes(bucket, frame_path, buf.tobytes(), "image/jpeg")
+                frames_info.append(FrameInfo(
+                    frame_index=frame_idx,
+                    time_sec=time_sec,
+                    frame_path=frame_path,
+                ))
+                upload_index += 1
+
+            frame_idx += 1
+
+        cap.release()
+
+        logger.info("extract_frames_done", extra={
+            "job_id": req.job_id,
+            "frames": len(frames_info),
+        })
+        await _post_callback(callback_url, {
+            "job_id": req.job_id,
+            "status": "SUCCESS",
+            "frames": [f.model_dump() for f in frames_info],
+            "output_path": None,
+            "error_message": None,
+        })
+
+    except Exception as exc:
+        logger.error("extract_frames_failed", extra={"job_id": req.job_id, "error": str(exc)})
+        await _post_callback(callback_url, {
+            "job_id": req.job_id,
+            "status": "FAILED",
+            "frames": None,
+            "output_path": None,
+            "error_message": str(exc),
+        })
+    finally:
+        if tmp and os.path.exists(tmp):
+            os.unlink(tmp)
+
+
+async def video_to_text_task(
+    req: VideoToTextRequest,
+    llm: LLMClient,
+    storage: StorageClient,
+    callback_url: str,
+) -> None:
+    cfg = get_config()
+    bucket = cfg["storage"]["buckets"]["source_data"]
+    sample_count = cfg["video"].get("frame_sample_count", 8)
+    model = req.model or cfg["models"]["default_vision"]
+
+    tmp = None
+    try:
+        video_bytes = await storage.download_bytes(bucket, req.file_path)
+
+        with tempfile.NamedTemporaryFile(suffix=".mp4", delete=False) as f:
+            f.write(video_bytes)
+            tmp = f.name
+
+        cap = cv2.VideoCapture(tmp)
+        fps = cap.get(cv2.CAP_PROP_FPS) or 25.0
+        start_frame = int(req.start_sec * fps)
+        end_frame = int(req.end_sec * fps)
+        total = max(end_frame - start_frame, 1)
+
+        # Uniform sampling
+        indices = [
+            start_frame + int(i * total / sample_count)
+            for i in range(sample_count)
+        ]
+        indices = list(dict.fromkeys(indices))  # deduplicate
+
+        content: list[dict] = []
+        for idx in indices:
+            cap.set(cv2.CAP_PROP_POS_FRAMES, idx)
+            ret, frame = cap.read()
+            if not ret:
+                continue
+            _, buf = cv2.imencode(".jpg", frame)
+            b64 = base64.b64encode(buf.tobytes()).decode()
+            content.append({"type": "image_url", "image_url": {"url": f"data:image/jpeg;base64,{b64}"}})
+
+        cap.release()
+
+        prompt = req.prompt_template or "请用中文详细描述这段视频的内容，生成结构化文字描述。"
+        content.append({"type": "text", "text": prompt})
+
+        messages = [{"role": "user", "content": content}]
+        description = await llm.chat_vision(model, messages)
+
+        # Upload description text
+        timestamp = int(time.time())
+        output_path = f"video-text/{req.source_id}/{timestamp}.txt"
+        await storage.upload_bytes(
+            bucket, output_path, description.encode("utf-8"), "text/plain"
+        )
+
+        logger.info("video_to_text_done", extra={"job_id": req.job_id, "output_path": output_path})
+        await _post_callback(callback_url, {
+            "job_id": req.job_id,
+            "status": "SUCCESS",
+            "frames": None,
+            "output_path": output_path,
+            "error_message": None,
+        })
+
+    except Exception as exc:
+        logger.error("video_to_text_failed", extra={"job_id": req.job_id, "error": str(exc)})
+        await _post_callback(callback_url, {
+            "job_id": req.job_id,
+            "status": "FAILED",
+            "frames": None,
+            "output_path": None,
+            "error_message": str(exc),
+        })
+    finally:
+        if tmp and os.path.exists(tmp):
+            os.unlink(tmp)
diff --git a/tests/__pycache__/test_video_router.cpython-312-pytest-9.0.3.pyc b/tests/__pycache__/test_video_router.cpython-312-pytest-9.0.3.pyc
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diff --git a/tests/__pycache__/test_video_service.cpython-312-pytest-9.0.3.pyc b/tests/__pycache__/test_video_service.cpython-312-pytest-9.0.3.pyc
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index 0000000..6c753b4
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diff --git a/tests/test_video_router.py b/tests/test_video_router.py
new file mode 100644
index 0000000..703dc40
--- /dev/null
+++ b/tests/test_video_router.py
@@ -0,0 +1,71 @@
+import pytest
+from unittest.mock import AsyncMock, patch
+
+from app.core.exceptions import VideoTooLargeError
+
+
+def test_extract_frames_returns_202(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=10 * 1024 * 1024)  # 10 MB
+
+    with patch("app.routers.video.BackgroundTasks.add_task"):
+        resp = client.post(
+            "/api/v1/video/extract-frames",
+            json={
+                "file_path": "video/test.mp4",
+                "source_id": 10,
+                "job_id": 42,
+            },
+        )
+    assert resp.status_code == 202
+    data = resp.json()
+    assert data["job_id"] == 42
+
+
+def test_extract_frames_video_too_large_returns_400(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=300 * 1024 * 1024)  # 300 MB > 200 MB
+
+    resp = client.post(
+        "/api/v1/video/extract-frames",
+        json={
+            "file_path": "video/big.mp4",
+            "source_id": 10,
+            "job_id": 99,
+        },
+    )
+    assert resp.status_code == 400
+    assert resp.json()["code"] == "VIDEO_TOO_LARGE"
+
+
+def test_video_to_text_returns_202(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=10 * 1024 * 1024)
+
+    with patch("app.routers.video.BackgroundTasks.add_task"):
+        resp = client.post(
+            "/api/v1/video/to-text",
+            json={
+                "file_path": "video/test.mp4",
+                "source_id": 10,
+                "job_id": 43,
+                "start_sec": 0,
+                "end_sec": 60,
+            },
+        )
+    assert resp.status_code == 202
+    assert resp.json()["job_id"] == 43
+
+
+def test_video_to_text_too_large_returns_400(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=300 * 1024 * 1024)
+
+    resp = client.post(
+        "/api/v1/video/to-text",
+        json={
+            "file_path": "video/big.mp4",
+            "source_id": 10,
+            "job_id": 99,
+            "start_sec": 0,
+            "end_sec": 60,
+        },
+    )
+    assert resp.status_code == 400
+    assert resp.json()["code"] == "VIDEO_TOO_LARGE"
diff --git a/tests/test_video_service.py b/tests/test_video_service.py
new file mode 100644
index 0000000..3e33483
--- /dev/null
+++ b/tests/test_video_service.py
@@ -0,0 +1,195 @@
+import io
+import json
+import os
+import tempfile
+import pytest
+import numpy as np
+import cv2
+from unittest.mock import AsyncMock, MagicMock, patch
+
+from app.models.video_models import ExtractFramesRequest, VideoToTextRequest
+
+
+def _make_test_video(path: str, num_frames: int = 10, fps: float = 10.0, width=64, height=64):
+    """Write a small test video to `path` using cv2.VideoWriter."""
+    fourcc = cv2.VideoWriter_fourcc(*"mp4v")
+    out = cv2.VideoWriter(path, fourcc, fps, (width, height))
+    for i in range(num_frames):
+        frame = np.full((height, width, 3), (i * 20) % 256, dtype=np.uint8)
+        out.write(frame)
+    out.release()
+
+
+# ── US3: Frame Extraction ──────────────────────────────────────────────────────
+
+@pytest.fixture
+def frames_req():
+    return ExtractFramesRequest(
+        file_path="video/test.mp4",
+        source_id=10,
+        job_id=42,
+        mode="interval",
+        frame_interval=3,
+    )
+
+
+@pytest.mark.asyncio
+async def test_interval_mode_extracts_correct_frames(mock_storage, frames_req, tmp_path):
+    video_path = str(tmp_path / "test.mp4")
+    _make_test_video(video_path, num_frames=10, fps=10.0)
+
+    with open(video_path, "rb") as f:
+        video_bytes = f.read()
+
+    mock_storage.download_bytes = AsyncMock(return_value=video_bytes)
+    mock_storage.upload_bytes = AsyncMock(return_value=None)
+
+    callback_payloads = []
+
+    async def fake_callback(url, payload):
+        callback_payloads.append(payload)
+
+    with patch("app.services.video_service._post_callback", new=fake_callback):
+        from app.services.video_service import extract_frames_task
+        await extract_frames_task(frames_req, mock_storage, "http://backend/callback")
+
+    assert len(callback_payloads) == 1
+    cb = callback_payloads[0]
+    assert cb["status"] == "SUCCESS"
+    assert cb["job_id"] == 42
+    # With 10 frames and interval=3, we expect frames at indices 0, 3, 6, 9 → 4 frames
+    assert len(cb["frames"]) == 4
+
+
+@pytest.mark.asyncio
+async def test_keyframe_mode_extracts_scene_changes(mock_storage, tmp_path):
+    video_path = str(tmp_path / "kf.mp4")
+    # Create video with 2 distinct scenes separated by sudden color change
+    fourcc = cv2.VideoWriter_fourcc(*"mp4v")
+    out = cv2.VideoWriter(video_path, fourcc, 10.0, (64, 64))
+    for _ in range(5):
+        out.write(np.zeros((64, 64, 3), dtype=np.uint8))        # black frames
+    for _ in range(5):
+        out.write(np.full((64, 64, 3), 200, dtype=np.uint8))    # bright frames
+    out.release()
+
+    with open(video_path, "rb") as f:
+        video_bytes = f.read()
+
+    mock_storage.download_bytes = AsyncMock(return_value=video_bytes)
+    mock_storage.upload_bytes = AsyncMock(return_value=None)
+
+    callback_payloads = []
+
+    async def fake_callback(url, payload):
+        callback_payloads.append(payload)
+
+    req = ExtractFramesRequest(
+        file_path="video/kf.mp4",
+        source_id=10,
+        job_id=43,
+        mode="keyframe",
+    )
+    with patch("app.services.video_service._post_callback", new=fake_callback):
+        from app.services.video_service import extract_frames_task
+        await extract_frames_task(req, mock_storage, "http://backend/callback")
+
+    cb = callback_payloads[0]
+    assert cb["status"] == "SUCCESS"
+    # Should capture at least the scene-change frame
+    assert len(cb["frames"]) >= 1
+
+
+@pytest.mark.asyncio
+async def test_frame_upload_path_convention(mock_storage, frames_req, tmp_path):
+    video_path = str(tmp_path / "test.mp4")
+    _make_test_video(video_path, num_frames=3, fps=10.0)
+    with open(video_path, "rb") as f:
+        mock_storage.download_bytes = AsyncMock(return_value=f.read())
+    mock_storage.upload_bytes = AsyncMock(return_value=None)
+
+    callback_payloads = []
+    async def fake_callback(url, payload):
+        callback_payloads.append(payload)
+
+    req = ExtractFramesRequest(
+        file_path="video/test.mp4", source_id=10, job_id=99, mode="interval", frame_interval=1
+    )
+    with patch("app.services.video_service._post_callback", new=fake_callback):
+        from app.services.video_service import extract_frames_task
+        await extract_frames_task(req, mock_storage, "http://backend/callback")
+
+    uploaded_paths = [call.args[1] for call in mock_storage.upload_bytes.call_args_list]
+    for i, path in enumerate(uploaded_paths):
+        assert path == f"frames/10/{i}.jpg"
+
+
+@pytest.mark.asyncio
+async def test_failed_extraction_sends_failed_callback(mock_storage, frames_req):
+    mock_storage.download_bytes = AsyncMock(side_effect=Exception("storage failure"))
+
+    callback_payloads = []
+    async def fake_callback(url, payload):
+        callback_payloads.append(payload)
+
+    with patch("app.services.video_service._post_callback", new=fake_callback):
+        from app.services.video_service import extract_frames_task
+        await extract_frames_task(frames_req, mock_storage, "http://backend/callback")
+
+    assert callback_payloads[0]["status"] == "FAILED"
+    assert callback_payloads[0]["error_message"] is not None
+
+
+# ── US4: Video To Text ─────────────────────────────────────────────────────────
+
+@pytest.fixture
+def totext_req():
+    return VideoToTextRequest(
+        file_path="video/test.mp4",
+        source_id=10,
+        job_id=44,
+        start_sec=0.0,
+        end_sec=1.0,
+    )
+
+
+@pytest.mark.asyncio
+async def test_video_to_text_samples_frames_and_calls_llm(mock_llm, mock_storage, totext_req, tmp_path):
+    video_path = str(tmp_path / "totext.mp4")
+    _make_test_video(video_path, num_frames=20, fps=10.0)
+    with open(video_path, "rb") as f:
+        mock_storage.download_bytes = AsyncMock(return_value=f.read())
+    mock_llm.chat_vision = AsyncMock(return_value="视频描述内容")
+    mock_storage.upload_bytes = AsyncMock(return_value=None)
+
+    callback_payloads = []
+    async def fake_callback(url, payload):
+        callback_payloads.append(payload)
+
+    with patch("app.services.video_service._post_callback", new=fake_callback):
+        from app.services.video_service import video_to_text_task
+        await video_to_text_task(totext_req, mock_llm, mock_storage, "http://backend/callback")
+
+    assert callback_payloads[0]["status"] == "SUCCESS"
+    assert "output_path" in callback_payloads[0]
+    assert callback_payloads[0]["output_path"].startswith("video-text/10/")
+    mock_llm.chat_vision.assert_called_once()
+
+
+@pytest.mark.asyncio
+async def test_video_to_text_llm_failure_sends_failed_callback(mock_llm, mock_storage, totext_req, tmp_path):
+    video_path = str(tmp_path / "fail.mp4")
+    _make_test_video(video_path, num_frames=5, fps=10.0)
+    with open(video_path, "rb") as f:
+        mock_storage.download_bytes = AsyncMock(return_value=f.read())
+    mock_llm.chat_vision = AsyncMock(side_effect=Exception("LLM unavailable"))
+
+    callback_payloads = []
+    async def fake_callback(url, payload):
+        callback_payloads.append(payload)
+
+    with patch("app.services.video_service._post_callback", new=fake_callback):
+        from app.services.video_service import video_to_text_task
+        await video_to_text_task(totext_req, mock_llm, mock_storage, "http://backend/callback")
+
+    assert callback_payloads[0]["status"] == "FAILED"