Abstract: In view of problems that existing neutral point clamped (NPC) three-level inverter fault diagnosis method needs large amount of calculation and its accuracy is affected by noise and load, an open-circuit fault diagnosis method of three-level inverter based on Park transformation was proposed. Park transformation is carried out on three-phase output current of the three-level inverter to calculate average current Park vector;sliding window is used to obtain a single-cycle current signal, and output current change is analyzed in case of IGBT fault according to inverter circuit structure; iterative method is used to determine threshold value of average current Park vector modulus, and 13 kinds of open-circuit faults of three-level inverter power tube are identifid and located combining phase angle and modulus.Simulation results show that the fault diagnosis accuracy of the method reaches 98.31%. Compared with traditional open-circuit fault diagnosis method, the proposed method does not need to learn and train the data of 13 kinds of open-circuit faults, which greatly reduces calculation amount and improves efficiency and accuracy of fault diagnosis.