In order to improve the reliability and effectiveness of the power line carrier communication, this paper proposes a signal processing method conducted at sending device terminal, i.e. phase adjustment precoding method. By using that the phase change of sending multiple carrier signal is different, it can change the superposition power of multiple carrier signal, thus reduce the peak-to-average ratio (PAR) of power line carrier communication. It analyzes the causes of the formation of PAR, and puts forward an optimization problem to solve each carrier sending phase by minimizing sending power as the objective function. Simulation results show that the precoding method can effectively restrain the high PAR problem of power line carrier communication and improve the reliability of the communication without any increase of complexity, which proves the correctness and effectiveness of the proposed method.