Compared with the traditional direct current (DC) transmission technology, the high-voltage DC transmission system based on the modular multi-level converter (MMC-HVDC), which features high-quality AC output voltage waveform and satisfying scalability, is suitable for multi-level voltage and power applications. Since the modulation strategy for MMC has a great effect on the MMC system’s output quality, the related study is necessary. First, the working principles of carrier phase shifted pulse width modulation (CPS-PWM) and three kinds of cascading methods for level-shifted PWM (LS-PWM), which can be applied to MMC, are introduced. Second, four kinds of modulation strategies, i.e., CPS-PWM, phase disposition pulse width modulation (PD-PWM), phase opposition disposition pulse width modulation (POD-PWM), and alternative phase opposition disposition pulse width modulation (APOD-PWM), are compared and analyzed in depth. Especially, the performances of these four modulation strategies are compared from aspects including the output phase voltage harmonic distortion rate and equivalent switching frequency, and their characteristics and application scenarios are summarized. Finally, a single-ended three-phase passive inverter MMC system is built in Matlab/Simulink to compare and analyze these four modulation strategies, and results verify the theoretical results.