The present application discloses a Three Dimensions (3D) video conversion system and method, and a key frame selection method and apparatus thereof. The key frame selection method includes: after the ith key frame is determined, calculating feature matching ratios of video frames after the ith key frame in a 2D video; selecting video frames each of which has a feature matching ratio within the range of T1˜T2 as candidate key frames; selecting a video frame from the candidate key frames as the (i+1)th key frame according to re-projection errors of the candidate key frames. Since key frames are selected according to the feature matching ratios of video frames to the previous key frame, and the later key frame is selected according to the re-projection errors of the candidate key frames. Thus the selection for key frames is more reasonable, and the 2D to 3D video conversion based on more reasonable key frames may improve converted 3D video quality.