A power allocation scheme based on generalized signal-to-leakage-and-noise ratio (SLNR) is proposed for cognitive radio network to address the problem of interference between cognitive network and primary network, therefore to guarantee quality of service of primary users (PUs) and improve perfor mance of secondary users (SUs). Beamforming matrix is introduced in the transmitter and the receiver of cognitive network to obtain a higher sum rate. Then a joint alternative iterative structure is developed to cascade transmit beamforming, power allocation and receive beamforming into an iteration loop. Simulation results show that the iteration loop converges fast and performs better than traditional beamforming scheme with equal power allocation in terms of the sum rate and the bit error rate.