In order to solve the problem that the stepped wave generated by the recent level modulation strategy has a large fitting error when approaching the sinusoidal modulation wave when the voltage level is not high and the number of levels is small, a compound modulation strategy is proposed. Firstly, the error between the stepped wave generated by NLM and the modulation signal is collected as the stepped modulation signal. Based on the principle of carrier modulation, a set of PWM pulse width modulation signals is generated by comparing the step modulation signals with triangular carrier signals whose amplitude is equal and phase is shifted sequentially. Finally, the step modulation signals are superimposed on the step modulation signals to form the driving signals. At the same time, the traditional voltage equalization algorithm is improved to reduce the switching frequency of the system.The simulation results of MMC built in Simulink show that the quality of output voltage waveform is guaranteed, the switching loss and total current distortion are reduced, and the improved voltage equalization algorithm can effectively suppress the voltage fluctuation of submodule capacitors and improve the conversion efficiency of MMC system.