The open-circuit fault in an inverter is a common fault in an Uninterruptible Power Supply (UPS) system. It can reduce power quality and even cause potential secondary faults in other components. It is of great significance to study effective fault diagnosis technology of inverters to help improve the reliability of a UPS system. Therefore, the Park vector method based on absolute normalization is used for open-circuit fault detection of the inverter. The normalized value between the average value of phase current and the average value of the absolute value of phase current is used to diagnose and locate an open-circuit fault of a single or two switches. The normalized value between the average value of the absolute value of phase current and the Park vector module is used to diagnose the open-circuit fault of two switches of the same bridge arm. This method can effectively realize fault detection and fault location. Finally, the proposed fault diagnosis method is verified using Matlab/Simulink. The simulation results show the effectiveness of the proposed fault diagnosis method. This work is supported by National Natural Science Foundation of China (No. 51507002) and Science and Technology Project of Guangxi Power Grid Co., Ltd. (No. GXKJXM20180433).