We research a bi-directional traffic flows network which two nodes are in each signal coverage area's in this article. Selecting optimum relaying using opportunistic relaying strategy based on instantaneous channel state information, then judging whether needs optimum relaying participates in the cooperation according to the threshold value. Don't need relay for transmission when the direct-transmission channel state is bigger than the threshold value; whereas sources and optimum relaying cooperate together. Comparing with direct transmission model, this strategy improves the network throughput, enhances the performance against fading, and reduces system symbol-error-rate effectively in high signal-to-noise ratio.