Abstract: The traditional voltage balancing bubble sorting algorithm of MMC-HVDC sub modules has the disadvantages of high time complexity and frequent switching of submodules. To this end,this paper proposes an MMC-HVDC submodules capacitor voltage balance algorithm that combines the bucket sorting algorithm and the submodules capacitor voltage dispersion threshold. The voltage dispersion threshold is introduced,and the capacitor voltage sequence of the current period is compared with the threshold value to form three groups of high voltage,medium voltage and low voltage. Second,the three groups of voltage sequences are divided according to the idea of bucket sorting. Finally, the switching order of submodules in different sections is determined according to the bridge arm current direction. In this thesis, the proposed algorithm is simulated by MATLAB and PSCAD/EMTDC,and the effectiveness of the method in reducing the time complexity and power module switching frequency is verified.