MMC is widely used in high-voltage frequency conversion and high-voltage direct current transmission because of its sub-module cascading, high output level and good waveform quality. Firstly, the basic topology and working principle of three-phase MMC are introduced. On the basis of the recent level approximation modulation method, the traditional capacitor voltage sequencing method is improved, and the voltage deviation reference quantity and input between the same bridge arm sub-module are introduced. The module capacitor voltage sorting coefficient avoids the frequent switching of the IGBT of the same sub-module due to the sorting algorithm, and reduces the switching loss of the converter. The 11-level MMC simulation model is built by Matlab/Simulink, and the relationship between the maximum voltage deviation between the same bridge arm sub-module and the IGBT switching power loss and the voltage deviation reference quantity between sub-modules and different sorting coefficients is given. The contradiction between the two is transformed into a multi-objective optimization model. An optimal control algorithm with weighting coefficients is designed. The optimal parameters with weighting coefficient of 0.8 are given. The simulation is verified under the optimal parameters. The simulation results show the effectiveness of the optimal control algorithm..