The invention discloses a control method for reducing the number of MMC (Modular Multilevel Converter) bridge arm sub-modules. By injecting an additional modulation signal into a valve-side voltage modulated wave, a maximum voltage value that each bridge arm needs to output is reduced, the number of the sub-modules required for each bridge arm is reduced while ensuring that the output rated voltage of each sub-module is constant, thereby reducing the volume and investment cost of an MMC converter valve. By adopting a control strategy of the invention, under the condition that the average voltage borne by a switching device is kept unchanged, the number of the sub-modules required for an MMC bridge arm is reduced without increasing the volume of the sub-modules and the operation loss of anMCC.