Study on the subject of suppressing the capacitor voltage fluctuation of modular multilevel converter (MMC) is scarce, thus this paper proposes a control strategy for suppressing the double fundamental frequency fluctuation of capacitor voltage of MMC. The sum of capacitor voltages of phase unit reflects the double fundamental frequency fluctuation of submodule capacitor voltage, so the strategy uses double fundamental frequency negative sequence rotational frame to decompose the capacitor voltages of three phase units to dq0 components. An additional controller is designed to control the dq0 components, which suppresses the double fundamental frequency fluctuation of capacitor voltage of submodules. The MMC simulation model containing the additional controller is established based on the Matlab/Simulink. The results show that the phase capacitor voltages controller can suppress the double fundamental frequency fluctuation of submodule capacitor voltage efficiently without increasing the submodule capacitance. It is also shown that this additional controller doesn’t affect the AC voltage and current output by external MMC.