Abstract: Reducing the amplitude of the capacitor voltage ripple of the sub-module in the multilevel modular converter（MMC） is expected to reduce the capacitance, volume and cost of the sub-module capacitor, and has important research value. This paper analyzes the characteristic of the capacitor voltage fluctuation and the fluctuation difference between half-bridge and full-bridge sub-modules with over-modulation in hybrid MMC, which determines the corresponding energy accumulation interval. Then, the extreme point distribution and zero crossing point distribution of the arm voltage reference with the third harmonic voltage injection are discussed. The relationship among the injection coefficient of third harmonic voltage, the modulation index and the fluctuation difference suppression capability is established. Aiming at reducing the difference of capacitance energy accumulation between half-bridge and full-bridge sub-modules, the third harmonic voltage injection quantity is optimized, and the suppression strategy of capacitor voltage fluctuation difference based on the third harmonic injection optimization is proposed. Then an example of calculation for reducing the capacitance is presented. Finally, the correctness and validity of the proposed strategy are verified by the simulation of double-end hybrid MMC in PSCAD/EMTDC.