In this paper, a distributed power control technique is proposed for Ad hoc network composed of secondary users (SU) within a single-hop cognitive radio Ad hoc network (CRAHN) application scenario in Interweave mode to maximize the capacity of the secondary network. Once authorized spectrum becomes idle, SUs in the secondary network will use authorized spectrum for concurrent communication, which aims at optimizing the transmission power of each SU so as to optimize the network spectrum efficiency. We first introduce the optimization approximation model of network capacity according to the given application scenario. In order to solve the non-convex problem, we establish the equivalent game model based on the network capacity optimization problem. Then we prove the existence and uniqueness of Nash equilibrium under the condition of different SINR. Finally, we propose a distributed power control algorithm based on Gradient Play method to realize optimal resource allocation. Simulation results show that the proposed algorithm can support a certain number of concurrent communication users and improve the spectral efficiency of the network system to a certain degree.