The invention discloses a UPQC starting control method based on an MMC in the technical field of electric power system flexible power transmission and distribution and electric power electronics. The method has the technical scheme that through three provided periods including the uncontrolled rectifying period, the UPQC parallel side direct constant current voltage period and the UPQC serial side modulation voltage boosting period according to slope function, serial and parallel side sub modules of the UPQC based on the MMC are subjected to successive voltage boosting so that no impact exists in the voltage boosting process, the safety of the sub modules and the corresponding equipment is ensured, in addition, the unified consideration is realized with the modulation algorithm, the realization is easy, furthermore, the whole UPQC system based on the MMC can be smoothly put into an electric network, the starting process and the normal modulation process realize the stable transition, and the impact phenomenon caused by control mode conversion cannot occur.