The threephase PWM inverter, as the research object, the failure characteristics of IGBT open circuit fault was in depth studied. By comparing the normal and fault between the signal and the phase voltage, integral algorithm based on phase voltage is established. And the diagnosis and orientation method is put forward. Finally, the threephase bridge inverter model is constructed in Matlab / Simulink, through the simulation of the circuit to the method of the research, we can see that the method used in this paper can be very accurately to get the fault location from the results.