Abstract: In cooperative communication system, traditional optimal relay selection method was only based on the channel state information (CSI) of legitimate users, however, in the actual transmission system, the security of the system would be greatly reduced in the presence of potential eavesdroppers. The optimal relay selection (ORS) method has improved the performance of the system when the CSI of the eavesdropper was taken into account, but the transmit power between source and relay was also allocated equally. For amplify-and-forward protocol, a power allocation factor which is according to the CSI between source and relay and between relay and destination is proposed to minimize the Secrecy Outage Probability (SOP) when the total transmit power is constrained. Simulation results show it can reduce the SOP and improve the performance of the system when the transmit power is allocated appropriately.