Abstract: We propose a full-duplex relay selection algorithm based on amplify-and-forward protocol and conduct the theoretical analysis in the cooperative communication system with multiple relays. Because of the existence of the loop interference in duplex relay, to guarantee the quality of communications, we select a relay based on maximizing the marginal signal to interference and noise ratio first, and then alleviate the impact of loop interference on the system by using the optimal amplify factor of relay. The relay selection algorithm based on the optimal amplify factor can provide a better performance than the traditional algorithm maximizing the marginal signal to interference and noise ratio in terms of outage probability, power consumption of relay, and received signal to interference and noise ratio at the destination. Finally，we demonstrate the advantages of the proposed algorithm with Monte Carlo simulations.