Adopting the moving object detection algorithm of moving background, an improved three-step search algorithm is proposed. The proposed algorithm can effectively accelerate motion-vectors search and reduce computational complexity while keeping up high search accuracy. Firstly，the same background of three continuous images are fixed to the same position of the same image through the global motion compensation. Secondly, three continuous edge images information are obtained by edge extracting, and the motion information is detected by three-frame-difference operation. Finally, morphologic close-algorithm and morphologic fill-algorithm are used to extract the moving objects. Experimental results demonstrate that this method can accurately extract moving objects, and has strong robustness.