The invention relates to a method for reducing a capacitance value of a modular multilevel converter, and belongs to the technical field of voltage source converters and the technical field of direct current transmission. According to the method disclosed by the invention, a capacitance voltage direct-current component operation value of a sub-module is properly reduced to a certain ratio relative to a capacitance voltage direct-current component rated value of the sub-module; meanwhile, the quantity of bridge arm cascade sub-modules and the capacitance voltage peak value of the sub-module are kept invariable, so that the maximal allowed amplitude of the capacitance voltage fluctuation of the sub-module is increased. The larger the maximal allowed amplitude of the capacitance voltage fluctuation of the sub-module is, the smaller the required capacitance of the sub-module is, so that the sub-module can select a relatively small capacitance value. The cost and the size of a capacitor in the modular multilevel converter both take up a great proportion on the whole, so that the method disclosed by the invention has significant benefits on reduction of the cost and the size of the modular multilevel converter.