Abstract: Modular multi-level converter (MMC) has such characteristics as multi-variable and strongly nonlinear. From the perspective of energy dissipation and simple structure, the bilinear Lagrangian equation in a-b-c synchronous reference frame is developed directly based on the dynamic mathematical model of MMC to strictly analyze passive features of the system. Then energy function for balancing capacitor voltage is designed. By combining the simple architecture of PI control, a novel passivity-based PI control method is proposed to ensure that the desired fast trajectory tracking can be realized under the premise of global asymptotic stability. A simulation model of MMC grid-connected system is built up by using MATLAB/SIMULINK, and the simulation results show that the proposed control method has such advantages as quick response, high stability and strong robustness.