The invention discloses a physical layer communication method and device of an MIMO power line communication system. The physical layer communication method comprises the steps of S1, adding a pre-coding matrix and a decoding matrix to a transmitting terminal and a receiving terminal, respectively and estimating a mean square error of transmission signals by utilizing the channel matrixes betweenthe transmitting terminal and the receiving terminal; and S2, solving the pre-coding matrix and the decoding matrix based on minimization of the sum of communication mean square errors of all legal users under the precondition of guaranteeing that the mean square error of the transmission signals between the transmitting terminal and an eavesdropper terminal is greater than a set minimum mean square error at the eavesdropper terminal; and S3, coding useful signals at the transmitting terminal by utilizing the solved pre-coding matrix, and decoding received signals at the receiving terminal byutilizing the solved decoding matrix. The physical layer communication method and device of the MIMO power line communication system suppress threats of eavesdroppers to power line communication and improve the communication security on the basis of guaranteeing communication of legal users efficiently.