The sub-module's（SM's）redundant configuration of modular multilevel converters（MMCs）is one of the technical difficulties that need to be resolved by the MMC engineering applications.The structure of an MMC and its SM is described，and the reliability of the SM and the MMC is analysed as well.An effective SM redundancy configuration calculation method for MMCs is proposed.The method offers the two SM engineering redundant reference values of the MMC，and when the number of SM redundant configuration is between the two redundant reference values，the increase in the number of redundant SM enables a faster increase in the reliability of the MMC.The redundancy and reliability index is defined based on the above method.The index measures the reliability efficiency of different MMC's redundant SM.The justifications and effectiveness of the proposed MMC sub-module redundancy configuration calculation method and the defined index are proved through engineering applications.