According to the open-circuit fault of each phase current characteristics, taking fault current as the fault information, this paper proposes a switch open-circuit fault diagnosis method based on extremum ratio of three-phase current and probabilistic neural network for the three-phase PWM inverter.By this method takes the output current of extremum ratio as the fault characteristic, using the nonlinear mapping classification ability of the probabilistic neural network to realize the fault diagnosis of inverter.The simulation results show that the method can accurately detect and locate failure power switch and has good robustness for noise.