The main factors that have effect on image contrast of extended objects in atmosphere were analyzed.The influence of tilt removal to the image contrast was mathematically investigated, and the optimization of using the absolute difference algorithm for obtaining the tilt was studied.Tilt removal of imaging beam by using a fast steer mirror could enhance the image contrast of extended object, facilitate the later digital image processing, and reduce the time cost for target recognition and tracking, thus it is valuable for improving the photoelectric tracking efficiency of low-contrast extended object.