Although the application of modular multilevel converter (MMC) is becoming more and more widespread, due to its working mode, there will be low-frequency ripple voltages in the sub-module (SM) capacitors. The most significant ones are the fundamental frequency ripple and the double frequency ripple, which will undoubtedly increase the volume of the SM capacitors and affect the application of MMC. This paper intends to use power decoupling technology to solve this problem by adding an energy storage inductor; the ripple of the SM capacitor voltage can be greatly reduced. Simulation and experimental studies are implemented, and the results confirm the effectiveness of the proposed strategy.