According to the feature of radar/ARPA simulator, this paper presents a novel algorithm of objects detection in automatic alert circle using modern image processing technology. The algorithm turns two echo images of consecutive scanning period into two binary images, and then uses logical operation and the method of region growing to detect objects in automatic alert circle. The algorithm doesn’t need complex operations, and filters out the detecting interference of the noise. Experiments show that the algorithm can be used in objects detecting of radar/ARPA simulator.