Abstract: With the modular multilevel converter (MMC) widely applicable in different engineering fields, the IGBT open circuit fault has become increasingly prominent, and it has become the main factor restricting its stable operation. The IGBT open-circuit fault diagnosis and localization method (FDL), as the main means to identify faults, can guarantee the continuous operation of MMC. First, this paper introduces the basic structure of MMC. At the same time, the characteristics of MMC under IGBT open-circuit fault are analyzed, where the current path of the faulty sub-module (SM) and the changing trend of capacitor voltage are clarified. Then, the main literatures are classified according to the means used to locate faulty IGBTs, and the advantages and disadvantages of different FDL methods are summarized. Finally, the future development and trend of MMC open circuit fault diagnosis and location methods are prospected. The main problems and ideas that should be solved in hardware-based, model-based and artificial intelligence-based diagnosis schemes in reducing hardware complexity, improving fault diagnosis accuracy and reducing computational load are pointed out.