The modular multilevel converter (MMC) had been widely studied and applied in the medium and high voltage direct current (HVDC) transmission system. The existence of internal circulation was an important phenomenon in MMC. According to the mechanism of MMC topology and its internal circulation, its threephase circulation was divided into 2 DC components based on negativesequence coordinate transformation of double fundamental frequency. And a circulating current suppressing controller was designed. The simulation results showed that the cotroller could effectively suppress the circulation of arm current and reduced the distortion of waveform without negative impacts on MMC external output AC voltage and current.