Abstract: Due to favorable tolerance of environment and focusing of energy in time reversal,a single monostatic trans-ducer with active time reversal signal can be used to detect targets of low signal to noise ratio.With the acoustic multi-path model based on classic ray theory,the feasibility of active TR signal detection is conducted theoretically.Through the detailed analysis of the active TR processing gain of a single monostatic transducer,the approximately proportional relationship between processing gain and the number of multipath is developed,which provides a theoretic basis for weak signal detection.In the condition of acoustic multipath propagation,the theory proposed in this paper is verified by the simulation of the active TR target detection with CW signal.