Abstract: In Dynamic Environment（DE），the number of active users，their locations，as well as the parameters that characterize their channel states，vary with time.Proved by several authors，identifying active users and using this information will cause significant performance improvement of Multiuser Detection（MUD）.By using the Random Set Theory（RST），a MUD in DE based on Particle Swarm Optimization（PSO） is presented.Simulation results show that the detector has good performance on optimization and efficient adaptability.