To resolve the problem of the dim targets detection for shipboard infrared search and track system target image，an algorithm called outlier-based targets detection(OBTD)is presented.According to the different characteristic between targets and background,dim target can be considered as outlier.Divide the origin image into small images;Calculate the local-reachable-density for every pixel;after that,the outlier can be found by setting threshold and the possible targets are obtained.The algorithm merit lie in a good real-time because of a low time complexity.The experiment shows that the algorithm can detect the dim target effectively,and it can offers convenience to further target recognition and track.