The capacity of existing two-level and three-level converters is limited, and the unique cascading expansion ability of modular multilevel converter (MMC) makes it meet the requirements of high-power and high-voltage photovoltaic power generation sites. The combination of MMC into photovoltaic power generation system has become a new trend. Compared with the new topology of the two-stage two-level and three-level photovoltaic inverter system, the new topology of the photovoltaic module directly combined with the MMC sub-module has less DC/DC link, which greatly improves the capacity and voltage of the grid-connected system, but the stability of the system is greatly reduced. In particular, the voltage imbalance of the maximum power point and the voltage instability of the MMC submodule in the condition of local shadow become a difficult problem. To solve this problem, the modulation strategy of the submodule is studied, and an independent modulation strategy that is not affected by the MMC output is proposed. The relationship between the capacitor voltage of the submodule and the output of the MMC is analyzed, and the voltage of the submodule is stabilized at the maximum power point voltage of the PV module. Finally, the simulation verifies that the modulation strategy is feasible. The results show that the control strategy not only improves the photovoltaic utilization rate, but also plays a role in voltage regulation protection for the MMC sub-module, and the whole photovoltaic grid-connected system has less harmonic pollution to the power grid.