Abstract: To improve the operational reliability of modular multilevel converter(MMC), the MMC arm is usually equipped with redundant submodules. Based on redundant MMC, the fault tolerance strategy of hot and cold hybrid standby is improved. The control complexity and power loss of MMC with different fault tolerance strategies are analyzed and compared. In order to complete the hybrid standby fault-tolerant strategy, a charging control strategy for the cold standby sub-module of MMC is proposed based on the nearest-level modulation(NLM) principle, which reduces the influence of the cold standby sub-module charging process on the output performance of MMC. In addition, the method of setting the number of hot and cold standby submodules is proposed. Finally, a 21-level three-phase MMC model is built based on the PSCAD/EMTDC simulation platform. The simulation results show that the improved mixed-reserve fault-tolerant strategy can effectively reduce the fluctuation of the fault-tolerant transition process, and verify the effectiveness and feasibility of the fault-tolerant control strategy.