Abstract: In order to improve the reliability of symmetrical six-phase permanent magnet synchronous motor system, a logical dynamical model of the motor drive system is constructed. On this basis, a nine-switch converter fault diagnosis method is proposed. The method uses the logic dynamic model to provide the voltage priori information and compare with the actual voltage output value containing fault information, extracts the fault information, so that to accurately locate switch fault. The simulation results confirm the effectiveness of the proposed method.