By analyzing upper and lower bridge arm current component of modular multilevel converter and characteristic of sub-module capacitor voltage ripple, and calculating the doubling-frequency current of MMC capacitor, this paper proposes an algorithm for suppressing the capacitor voltage ripple through injecting doubling harmonic current in the arm bridge current of MMC. The doubling harmonic current injection controller is designed to control the value of the current and get the optimal doubling harmonic thus to minimize the capacitor voltage fluctuation of sub-module. The capacitor voltage ripple range is compared when the suppressing circulating current is zero and second harmonic current is injected by using the simulation results based on PSCAD environment, and the capacitor voltage harmonic distortion rate is analyzed. The results verify the validity and efficiency of the algorithm.