Abstract: Modular multilevel converter (MMC) has become an effective solution for new all-solid-state special high-voltage power supply, and its lightweight design to save equipment space cost has become a research hotspot. The primary factor limiting power density in MMC is the large size capacitance of submodule (SM), and to reduce the demand for the capacitance of the sub-module in modular multilevel converter, and increase system power density, an improved MMC(I-MMC) topology is proposed. Using isolated switched capacitor converters, a pair of SMs of the upper and lower arms are interconnected through a high frequency link. In the research, synchronous control was adopted on both sides of the high-frequency chain connecting the pair of SMs in the phase unit to make the SM capacitors present the characteristics of switched capacitors, realize free transfer of fluctuating power between the capacitors, and eliminate the fluctuation components of the oppostite phased fundamental frequency and triple frequency. Combining MMC operation modulation ratio and power factor, we analyzed the value of the SM capacitor after the fundamental frequency and triple frequency fluctuation components had been eliminated, and completed the modular design. This solution can reduce the sub-module capacitance to 1/4 of the conventional MMC's capacitance. Simulation and experimental results verify the correctness and the validity of the proposed topology scheme.