Modular multilevel converter (MMC),which is composed of a large number of submodules (SM) connected in series, is one of the most promising converters in high voltage field. The open-circuit fault and short-circuit fault of the SM's power switch seriously affect the reliability of the MMC,and it brings great challenges to the stable operation of the MMC. Therefore,in order to quickly detect the faulty SM,a fault detection method of the SM's power switch in the MMC based on Luenberger observer is proposed. Firstly,the fault characteristic of the SM is analyzed,and the model of Luenberger observer is established according to the mathematical relation of the SM capacitor voltage change. Then,the estimated capacitor voltage is calculated by Luenberger observer,and the detection of the power switch fault is realized by comparing the estimated capacitor voltage and the measured capacitor voltage. Finally,the simulation model of the MMC system based on the PSCAD/EMTDC and the experiment platform in the laboratory are respectively built. The simulation and experiment results show that the open-circuit and short-circuit fault of the SM can be detected correctly and effectively,which verifies the validity and feasibility of the proposed method,but each SM needs to be monitored.