Foreign object debris(FOD) on airport runways is a major hidden danger to the airports’ safety. So, quick and precise detection to FOD has practical significance. When the radar range is comparatively short, FOD reflection intensity changes with distance very severely. To solve this problem, this paper present a method of FOD detection based on image difference. Firstly, the amplitude of FOD is corrected based on the distribution characteristics of FOD gray value. Secondly, the FOD is segmented using local thresholds. Finally, targets are located by image block and target merging. Experimental results of real radar data show that the method is effective and realtime.