For specific military use of the environment, this paper study the performance change of the system used directional antennas and exploratory study the power optimization based on the symbol error rate(SER) under amplify-and-forward protocol (AF). Firstly, cooperative communication system model based on the directional antenna was established, and then the closed-form expression of the model was derived theoretically. Secondly, it analyzed the optimal power allocation (OPA) problem in order to minimize system SER. Finally, it analyzed and compared the performance of the SER combining with specific numerical and simulation in detail. The results showed that in the condition of using directional transmission and directional reception, system had smaller SER. Directional gain, channel quality and power distribution of each node had enormous impact on the whole system performance. OPA had better performance than equal power allocation (EPA).