The underwater detection sonar is applied to the semi-submersible oil production platform for real-time detection and early warning of illegal underwater frogman, stealth vehicle and alien underwater personnel. If necessary, the defense mechanism is adopted to drive the target away from the restricted waters. The underwater sonar detection scheme is designed in detail from three aspects: sonar image preprocessing, moving target detection and trajectory tracking. The application test of the detection scheme is carried out. The results show that the scheme can effectively detect the target and conduct trajectory tracking, which verifies the effectiveness of underwater detection sonar applied in the underwater security system of semi-submersible platform.