An improvement method is given to detect small weak targets. It predicts the background gray and the actual gray of the current pixel molded by two predicting windows. The method can not only solve the edge blur problem of the traditional method, but fit for the detection of the bipolar targets. It can perform at a high calculation speed, which is an important improvement in the filed. In the process of real-time missile-tracking. The reliability and stability of detection require that the method cannot only solve the edge blur problem of the traditional method, but work well with the background no matter it is bright or dark. According to the experiment using the images provided by DX-1 missile-tracking project, the method is proved effective.