Abstract: The multi-relay cooperative communication system model in rectangular tunnel based on multimode channel model was established, and the channel capacity calculation formulas of multi-relay cooperative communication system in amplify and forward mode and decode and forward mode were given. Under the circumstance of limited power in tunnel, optimal power allocation method of amplify and forward multi-relay cooperative communication system based on genetic algorithm and multiple KKT optimal power allocation method of multi-relay cooperative communication system in decode and forward were proposed to optimize the channel capacity. Simulation results show that the proposed optimal power allocation methods can improve channel capacity of the tunnel multi-relay cooperative communication system.