Abstract: In order to control the DC voltage in modular multilevel converter（MMC） and reduce the switching loss,the structure of MMC and the principle of sub-module charging are introduced firstly,and it is pointed out that the DC voltage control in MMC includes two parts,the DC voltage control of system and sub-module.For the DC voltage control in system,its control theory is analyzed,and the corresponding control strategy is proposed.At the same time,for the high switching frequency caused by the blind action of switching devices in the sub-module control,an improved voltage balance control strategy for sub-module is proposed by the improvements of the traditional voltage sorting method.In the end,PSCAD/EMTDC simulation results show that DC voltage in MMC can run in the rated value steadily,and the switching frequency is reduced significantly.