For moving targets only taking a small proportion in a single frame image, while traditional edge detection method is too complicated to have a redundant calculation and is sensitive to the noise, it can not extract contour of the moving target obviously, so in this paper, an edge detection method combining the frame difference with image blocking is proposed. This method makes image difference firstly, and blocks the result with a certain threshold secondly, so as to extract the area of moving target. And thirdly, it detects the edge of this area, and finally does the “and” operation to the result of edge detection and frame difference to extract the target contour information. Experimental results show that the method can make edge detection with image blocking exactly. Also this method can detect the moving object accurately and effectively.