In multi-source multi-relay wireless cooperative sensor networks, the relay selection strategy based solely on the link quality usually leads to rapid exhaustion of the optimal relay energy resulting in inequality of node energy consumption and reduction of the entire network lifetime. Therefore, a selection strategy based on relaying residual energy is proposed. This strategy defines the inner product of the number of theoretically effective forwarding packets and the relaying residual energy. The selection criterion is to maximize the inner product. According to the simulation results, the proposed strategy can effectively balance the remaining energy of each relay node in the network, improve the total network throughput and extend the average network lifetime by about once at the cost about 10% of the average throughput.