The system losses and working performance of the modular multilevel converter（MMC）are directly affected by the harmonics in the circulating current.The inherent relation between the AC-side current，the leg current and the circulating current is first directly derived，which reveals that there always exists double-frequency harmonics whether under balanced or unbalanced grid conditions.Then，a novel circulating controller based on proportional-integral-resonant（PIR）control is proposed for effectively suppressing the harmonics in the circulating current，without the double-frequency negative-sequence transformation and the phase decoupling control.A MMC-HVDC simulation of a two-station model containing the PIR is created based on PSCAD/EMTDC.Simulation results have verified the availability of the control strategy when compared to the results from the circulating controller under double fundamental-line frequency negative sequence rotational frame.