The sub-module is an important part of the modular multilevel converter (MMC), its parameters are related to the safety and stable operation of the converter. The values of the sub-module capacitance parameters affect the DC voltage fluctuation of the system. Based on the steady state mathematical model of MMC, the analytical expression of the instantaneous power of the MMC bridge arm is obtained, and the relation expression of the sub-module capacitance energy is derived. By the method of mathematical calculation, this paper proves that the modular voltage is maximized when the converter is working in static var compensation mode, and the converter that can meet the voltage fluctuation rate requirements in the range of capacitance values under all working conditions is designed. The simulation is based on the HVDC Flexible demonstration project in Nanhui. The voltage fluctuation of the converter under different working conditions and the adaptability of the designed capacitance parameters to the converter operating conditions are verified.