Abstract: Based on the problem that key frame number can not adaptively adjusted according to the lens, the paper proposed an adaptively key frame extraction method based on frame difference and the motion information entropy. The method determined the number of extracted key frame according to changes of the lens and extracted key frame based on the maximal motion and changing information entropy. A mathematical model for key frame extraction was constructed. And the theory of this method was introduced in detail. Many contrasting experiment have demonstrated that this method is obviously better than the key-fame extraction by equal interval. By this method the changing process of sequence is represented perfectly. It provides better perception and understanding for the video.