To suppress the co channel interference in downlink multi cell systems, a novel zero forcing (ZF) precoding method is proposed based on the principle of orthogonal projection in linear algebra. A joint interference channel matrix is constructed, and then the channel matrix of the target user is projected onto the null sp ace of the joint matrix. Hence, the precoding matrix of the target user lies in the null space of all interference users without interfering with others. Compar ed with the conventional ZF method, the projection based method performs better at low signal to noise ratio (SNR). However, it is inferior to the conventional one at high SNR. Therefore, an adaptive ZF method is proposed combining the traits of both the conventional method and the projection based one. The computational complexities of the proposed methods are analyzed and compared with the conventional one. Simulation results show that the proposed adaptive ZF method offers better performance than the conventional one over a whole SNR regio n, and the complexity of the proposed method is a little higher than that of the convent ional one.