In order to realize auto target detection of multi-target image of shooting ranges, the similarity of targets' shape was utilized, and a new method for auto similar target detection based on shape entropy difference was proposed. First, the concept of shape entropy difference was presented, which merged shape information into the calculation of entropy, and the target was detected by searching for the extreme point of entropy difference. Then, an auto target detection method based on shape entropy difference was realized. Finally, the validity and robustness of the algorithm was approved by experiment. Compared with the common algorithms of target detection, the method proposed here has better tolerance capability against noise and illumination variety. The method can be used for target detection of image of shooting ranges and other similar applications.