Time reversal mirror(TRM) can use the physical characteristics of the underwater acoustic(UWA) channel to focus on the desired user in multi-user UWA communication. The active average sound intensity(AASI) detector can estimate all azimuths of users with the same frequency band at the same time in order to achieve directional communication by vector combination.Space-division multiple access(SDMA) based on TRM combined with the AASI detector is proposed in this paper, which can make the capacity of the code division multiple access(CDMA) UWA system significantly increase. The simulation and lake test results show that the 7-user UWA multi-user system can achieve low bit error communication. The active average sound intensity (AASI) detector can estimate all azimuths of users with the same frequency band at the same time in order to achieve directional communication by vector combination. Space-division multiple access (SDMA) based on TRM combined with the AASI detector is proposed in this paper, which can make the capacity of the code division multiple access ( CDMA) UWA system significantly increased. The simulation and lake test results show that the 7-user UWA multi-user system can achieve low bit error communication.