Abstract: The existing physical layer encryption algorithm, which is based on artificial noise, could affect legitimate receivers negatively when the number of users is no less than sending antennas in the multiuser MIMO system. In order to improve the multiuser MIMO system security under this scenario, this paper proposes a new multiuser MIMO system physical layer encryption algorithm based on joint channel state matrix. Firstly, multiple users are processed together, thus a multiuser joint channel state matrix is established. After Singular Value Decomposition (SVD) of the joint channel state matrix, the minimum singular value is obtained and can be utilized for pre-coding, so as to eliminate the interference of artificial noise to legitimate receivers. Further, the paper also presented an approach to optimize the power allocation. The simulation results show that the proposed algorithm can increase the secrecy capacity by 0.1 bit/(s?Hz) averagely, and improve the multiuser MIMO system security.