A thresholding segmentation algorithm for complex background is introduced. The method is based on edge detecting of infrared image. Firstly, the image edge is detected, and then segmentation threshold is computed by weighting and averaging value of pixels gray. The method is simple and practical, and it can segment the targets in the air and on the sea exactly. And it has been used in tracking system, the experimental result indicates that the algorithm is correct and real-time, and the system is reliable.