In order to solve the issue of increasing output harmonics and poor anti-interference ability after open-circuit fault of T-type inverter, an open-circuit fault diagnosis method of T-type three-level inverter based on voltage residual is proposed in this paper. Firstly, the working principle and open-circuit fault characteristics of T-type inverters are analyzed, and the state-space model of the system has been constructed via the space vector pulse width modulation strategy. Secondly, the line voltage detected by the actual sensor is contrasted with the reference value of the down-line voltage under normal conditions. The voltage residual signal is obtained through low-pass filtering processing. Compare the residual error with the fault detection threshold to determine whether the system is faulty. Then the logical relationship between the diagnostic variable and the fault switch tube is established to achieve the precise location of the inverter single tube fault. Finally, the proposed fault diagnosis method is verified by the MATLAB platform, and the simulation results show that the method is easy to implement, fast and reliable.