In recent years, voltage source HVDC transmission technology based on modular multilevel converter (MMC) has been widely used. In order to reduce the voltage fluctuation of the sub-module capacitor as much as possible without increasing the switching frequency, and reduce the harmonic content in the system output, a hybrid modulation strategy which can meet the requirements of HVDC transmission is proposed by combining the advantages of nearest level approximation modulation (NLM) and pulse width modulation (PWM). In this method, PWM is added on the basis of step modulation, and the error signal is modulated twice. The sub-module unit selection method is used to construct capacitor voltage average, balance control and current closed-loop control, so as to carry out real-time feedback correction for the switching state of each sub-module, so that the system can still control the voltage fluctuation of the sub-module capacitor and the output harmonic content at a lower switching frequency. Finally, the method is proved to be effective and feasible on the MATLAB/Simulink platform.