In this paper, we propose an improved fast detecting algorithm to solve the disadvantages which are the high computation, sensitive to light changes and the speed of moving object of the classical Gaussian mixture model. We extract the complete regions of the motion and the background by using frame different method of choosing the appropriate space. The algorithm reduces the amount of calculation because it just needs to calculate part of pixels. We use different background update rate in the different area of the background to respond the changes of the background timely. Finally, we introduce an environment mutation parameter to detect mutations. Through experiments, the algorithm has made a lot of improvement in the aspects of the instantaneity, the robustness and the stability. The moving target can be detected very well.