Abstract: A suppression strategy of circulating current of modular multilevel converter based on proportional complex integral controller was proposed according to double fundamental frequency negative sequence property of phase-to-phase circulating current of modular multilevel converter, which achieved circulating current suppression by setting resonant frequency of proportional complex integral controller to voltage frequency of power grid of double fundamental frequency negative sequence. In the strategy, there are no system parameters, no conversion of rotational coordinate and no current feedforward decoupling control in the proportional complex integral controller, so as to avoid the matter of poor control effect caused by changing system parameters. The simulation in Matlab/Simulink verifies correctness and effectiveness of the strategy.