Abstract: An orthogonal projection based SLNR joint transceiver design algorithm is proposed for cognitive MIMO system to realize effective spectrum sharing between cognitive user network and primary user network. The proposed algorithm can be implemented by two steps. Firstly, in order to eliminate the interference to primary user, a pre-processing that orthogonally project the transmitting signal to the null space of interfering channel was used at cognitive base station. Then, aimed at improving performance of the cognitive user network, an alternate iteration method was exploited to jointly design transceiver utilizing the modified SLNR as optimizational target. Simulation results demonstrate that proposed algorithm can efficiently suppress the co-channel interference and improve BER and sum-rate capacity performance of cognitive user network without disturbing the normal communication of primary user network.