When an open circuit fault occurs in the internal switching components of a modular multilevel converter （MMC）, it can cause distortion of the output current, increase inter phase circulating current, and seriously endanger the safe and stable operation of the equipment. Accurately identifying the submodules where open circuit faults occur in switch devices is crucial for the stable operation of MMC systems. The fault characteristics of IGBT and freewheeling diode open circuit faults are analyzed, and an IGBT open circuit fault diagnosis method based on the ratio of capacitance voltage increment is proposed to accurately identify the submodules where IGBT open circuit faults occur. An open circuit fault diagnosis method for freewheeling diodes is proposed based on the cumulative analysis of periodic fault characteristics, utilizing the relationship between bridge arm current and switching function when an open circuit fault occurs in a diode. This method accurately identifies the submodule where the diode open circuit fault occurs. And a redundant control scheme is designed. An MMC rectifier stage model is built based on the RTDS simulation platform, and the proposed fault diagnosis strategy can accurately determine the faulty components and submodules, solving the problem of difficult diagnosis of open circuit faults in switch devices.