Abstract: Underwater signal detection is the important supporting technology for passive sonar system to monitor targets, and the performance of signal detection directly affects the target detection capability of passive sonar system. This paper sort out the main techniques and achievements of passive sonar signal detection, which could be classified into two categories: the temporal information based signal detection technique and the spatial information based signal detection technique. Through the analyses of the existing problems and the application requirements, the potential developing direction of passive sonar signal detection is pointed out, that is, weak signal detection, false alarm probability reduction, and effective use of underwater acoustic propagation information in signal detection.