Abstract: In the event of a grid-side AC single-phase ground fault of the modular multilevel converter, the peak-to-peak value of the sub-module capacitor voltage fluctuation increases. If no measures are taken to suppress it, it will cause the sub-module over-voltage and endanger the safe operation of the devices. This paper proposes an optimized harmonic coupling injection strategy of double frequency circulating current and triple frequency voltage to reduce the capacitance voltage fluctuation of the MMC sub-module during the fault. This paper first establishes a mathematical model of MMC bridge arm power fluctuations for the coupling injection at the steady state and the coupling injection for the single-phase ground fault; Then, the principle and optimization process of the proposed harmonic coupling injection strategy are described in detail. By suppressing the three-phase bridge power fluctuation, the effect of reducing the capacitance voltage fluctuation is achieved; Finally, a simulation model is built on the PSCAD/EMTDC simulation platform to verify the effectiveness of the proposed strategy. The simulation results show that through the injection strategy proposed in this article, the capacitor voltage fluctuation of the sub-module during the fault period can be reduced by up to 33%, which can effectively avoid the sub-module capacitor overvoltage caused by the grid-side SLG fault, thus reducing the demand for SM capacitance.