Abstract: When modular multilevel converter(MMC) is operating at low frequency, there are relatively large low frequency capacitor voltage ripple, which causes the output voltage waveform distortion. Aiming at this problem, an improved high frequency injection strategy is proposed. Firstly, aiming at the problem that there is a certain deviation between the calculated value of module voltage and the actual value in the modulation process, the modulation method based on the actual value is selected to analyze the capacitor voltage fluctuation mechanism, and the basic principle of high-frequency injection is analyzed. In order to further optimize the amplitude of high-frequency circulation based on the existing high-frequency injection method, a high-frequency injection method combining square wave and trapezoidal wave is proposed. Finally, the proposed method is verified by Matlab/Simulink simulation. The results show that the proposed method can further optimize the high frequency circulation and the current amplitude of the bridge arm on the basis of realizing the low frequency voltage ripple suppression, and reduce the system loss.