Aiming at the problem of safe and reliable operation of three-phase quasi-Z-source inverters, a fast and practical IGBT open-circuit fault diagnosis method of quasi-Z-source inverters is proposed. The new IGBT open circuit fault diagnosis is realized based on the characteristics of the Z-source inverter. The main method is to observe the influence of the change of the shoot-through state before and after the fault on the system variables during the switching period to identify the fault. The implementation is divided into two consecutive stages, one is IGBT open-circuit fault detection and location, the second is to activate the redundant bridge arm to complete the fault-tolerant operation. The tests were carried out with a low-voltage prototype of a three-phase quasi-Z-source inverter. The experimental results verified that the designed IGBT open-circuit fault diagnosis scheme can quickly detect IGBT faults, and complete the system reconstruction within 0.5ms to achieve fault-tolerant operation. The use of the proposed Z-source inverter IGBT open-circuit fault diagnosis scheme in the rail transit system can ensure the reliable operation of the power supply system.