Abstract Being different from previous spectrum sensing research for single-antenna cognitive users, a spectrum sensing algorithm based on optimal power allocation and cooperative diversity under multi-antenna scenario is proposed. It used Sigular Value Decomposition(SVD) to implement power water-filling on each antenna based on channel condition, and the detection process of the cooperative spectrum sensing for the primary user is considered in the two-user network with multi-antenna. The ergodic capacity, detection probability and detection time in cognitive network with multi-antenna are analyzed with path-loss model based on exponential attenuation. Theory analysis and simulation results show that cognitive users with multi-antenna can not only make the cognitive network have maximal ergodic capacity, but also improve detection performance of the system through optimal power allocation and cooperative spectrum sensing.