Cooperative communication with spectrum sharing has been proved to be an efficient way to reduce energy consumption. In this paper, an optimal power allocation algorithm is proposed for mobile cooperative beamforming networks, where several Secondary Users (SUs) collaboratively act as relay nodes to assist the transmission of a Primary User (PU) in reward of transmitting their own information. The optimal power allocation scheme is obtained by solving a convex optimization problem aiming at minimizing network energy consumption with guaranteed Quality of Service (QoS) for both PU and SUs.