The harmonic magnetic potential waves of the permanent magnet synchronous motor(PMSM) are analyzed with the winding function method. It is proved that the injection of appropriate harmonic current can improve the output torque. a mathematical model of the nine-phase PMSM is established under the rotating coordinate system. By analyzing the composition of electromagnetic torque, the optimal current harmonic injection ratio is obtained. An approximate simplified calculation method of injection rate is proposed with the constraint of maximum power. The simulation model of the nine-phase vector control system is built. The changes of the output torque and current waveform of the motor before and after the harmonic injection are compared.