In the very recent years, the modular multilevel converter (MMC) topology has become the development trend of high-voltage high-power power conversion technologies. However, one specific issue associated with MMC is the necessity of very large capacitances, which stops it from reducing cost, increasing reliability, and improving power density. To reduce the capacitance of required capacitors, in this paper, the nature of the common-mode capacitor voltage fluctuation is analyzed, and the corresponding suppression scheme based on a proportional-integral plus resonant (PI+R) controller is proposed. Compared with conventional open-loop suppression methods, the control accuracy, flexibility, and robustness can be significantly improved. Finally, the validity and effectiveness of the proposed scheme are verified by simulation and experimental results.