Abstract: In this paper, the start-up process and control method of unified power quality controller (MMC-UPQC) based on modular multilevel converter are studied. Firstly, the basic operating principle and mathematical model of MMC-UPQC are described, and its start-up process is analyzed. Then, two different control methods are presented according to the characteristics of voltage boost process for convertor sub-module on series capacitor side, and their advantages and disadvantages are analyzed. In the end, through PSCAD/EMTDC software, the validity of start-up control method is verified, and simulation results show that there is low voltage current shock and smooth curve, and it is easy to realize during start-up process by using the control mode of sub-module rotation conduction for the up and down bridge arm on series converter side because of its progressively increasing voltage of module capacity, which is a better way to start-up.