According to the relevant standards of the international electrotechnical commission ( IEC), the utilization of pulse and sinusoidal voltages to determine the partial discharge inception voltage (PDIV) of the inverter motor insulation system is a key method to evaluate the insulation performance of low-voltage random-wound inverter-fed motors. However, the current IEC standard is difficult to identify the stator insulation weakness. To solve this problem, combines the PDIV off-line test technology under the repetitive pulse and the sinusoidal voltage, a three-step method is proposed to locate the weak point of the inverter-fed motor insulation. Firstly, the repeated pulse voltage is used to measure the PDIV of turn-to-turn insulation in the motor. Secondly, the PDIVs of phase-to-phase insulation and main wall insulation are measured by the sinusoidal voltage, which are compared with the turn-to-turn insulation PDIV. In this way, the weak point of the motor insulation can be determined. Finally, through the PDIV test in different grounding modes of the motor, it is possible to locate whether the weak point of insulation is located at the stator winding head. In addition, by simulation of stator voltage distribution, weak point identification of three commercial motors and corona life test of motors, the feasibility of the above method is comprehensively verified. The research can effectively locate the insulation weakness of the inverter-fed motor. And the efficiency of diagnosing potential risks in the motor insulation is improved.