Modular multilevel converter (MMC) generates circulating currents flowing between phases due to fluctuations in capacitor voltage, which increases energy loss and adversely affects the insulated gate bipolar transistors (IGBTs) in the sub-modules. In order to suppress the circulating current, a proportional controller capable of generating inverse doubling components is designed based on the dynamic mathematical model of the circulating current. The conventional quasi-resonant controller is improved for the high harmonics in the circulating current. And the proposed strategy is verified by simulation, the simulation results show that the proportional control strategy quickly reaches the steady-state suppression of the circulating current after only 0.2 s. The improved quasi-resonant control strategy optimizes the THD value by 3.56% compared with the pre-improvement one, which further reduces the high-frequency harmonic components and verifies the feasibility of the proposed strategy.