The invention provides a multi-user selection method of a full duplex both-way relay system based on instantaneous state information and a communication system. Two corresponding user scheduling methods are designed by use of a forwarding decoding protocol based on the case that the instantaneous system state information is available; one of the two scheduling methods possesses the lowest system implementation complexity, while the other one possesses the optimal interruption performance. In actual deployment, the corresponding scheduling methods can be selected according to different practical situations. The multi-user selection method is capable of effectively reducing the system interruption probability and solving the problem of too high interruption probability in the full duplex system; meanwhile, the equity between users can be ensured and the service quality of the system can be improved.