[Objective] To improve the reliability of the symmetrical six-phase permanent magnet synchronous motor (PMSM) drive system, this paper constructs a dynamic model based on a nine-switch converter and proposes an innovative open-circuit fault diagnosis method for the inverter. [Methods] Considering the dead-time effect of pulse width modulation, the logical mapping between the motor winding terminal-to-ground voltage and the switch state was defined. The dynamic correspondence between the phase voltage and the switch signal was established, and a unified logical model of the hybrid system was derived. Based on voltage hybrid logic operations, the voltage differences between standard and fault states were compared, and open-circuit faults were accurately located. [Results] The simulation results demonstrated that when an open-circuit fault occurred in the symmetrical six-phase PMSM inverter, the faulty switch could be accurately located through dynamic logic transformation and voltage difference analysis. [Conclusion] The proposed diagnostic method can effectively identify faults in a single switch or simultaneous open-circuit faults in two switches of the same bridge arm, demonstrating high accuracy and universality.