As the key equipment of UPFC,the reliability of the modular multilevel voltage sourced converter operation directly affects the safe and stable operation of the whole AC system.An k-out-of-n:G system model of Sunan 500 kV UPFC-MMC and the basic analysis method for it are proposed in this paper.The relation between the reliability of valve and different redundancies is clearly described.The reliability value and MTTF of Sunan 500 kV UPFC-MMC are evaluated.And the optimal amount of redundant sub-module is analyzed basing on the indices obtained in forenamed study.The reliability design optimization of MMC valve could meet the engineering requirements Sunan 500 kV UPFC and would provide a reference for follow-up project.