Hybrid modular multi-level converter (MMC) consisting can operate with a high modulation index,and it borrows the capacitor voltage balance method from half-bridge MMC. Thus,a strong coupling effect occurs in the charging behaviours of full bridge sub-modules (FBSMs) and half bridge sub-modules (HBSMs),which causes a low volatility of the ripple voltage of HBSMs' capacitors and challenges for its capacitance optimization. To address this issue,the ripple coefficients of HBSMs' and FBSMs' capacitor voltages are calculated for MMC operating with a high modulation index quantitatively. The optimize of HBSMs' capacitance can be clarified. Then,design principles of the optimal HBSMs' and FBSMs' average switching functions are explored. Hence,the optimization of HBSMs' capacitance is achieved with the constraint of the maximal value of sub-modules' instantaneous capacitor voltages. Further,under the conventional basic control framework of MMC,a control strategy based on module decoupling control which can regulate the dynamics of capacitor voltage accurately is proposed. Finally,hybrid MMC operating is modeled with MATLAB/Simulink. The proposed capacitor optimization method is verified. The simulation results show that the DC component and ripple component of capacitors' voltages can be regulated accurately,and the capacitance of HBSMs can be reduced effectively.