To improve the control effect of a modular multilevel converter (MMC) under traditional PI control, an MMC control strategy based on the Lyapunov direct method is proposed. First, the mathematical model of the MMC rectifier is established. Secondly, the Lyapunov energy function of the MMC system is constructed. Then, the control system is designed on the basis of the MMC mathematical model and energy function. The influencing factors of DC voltage stability are analyzed by the Nyquist stability criterion, and the control performance of Lyapunov strategy and PI control strategy are compared and analyzed by a Bode diagram in the classical frequency domain. Finally, the two control strategies are simulated and studied when the load is switched, the voltage amplitude of AC side changes suddenly and the three-phase voltage of AC side is unbalanced. The results show that the Lyapunov control offers better control than the PI, and it is more suitable for nonlinear and coupled strongly systems such as with an MMC.