The relays of opportunistic cooperative communication have different choice probabilities. Aiming at the problem of the unfair selection causes the reducing of network lifetime, a relay selection method based on proportional fairness is presented in this paper. This method is due to improve the average fairness and prolong the network lifetime. The relay which has the highest ratio in both instantaneous and average signal-to-noise is to be chosen as the cooperative relay. Under the quality requirement of system service, the main idea of the proposed scheme is that some relays with good instantaneous and low average channel condition may have more chance to forward the source signal. The simulation shows that, although the outage probability based on this relay selection method is reduced, the average fairness and the network lifetime are improved.