The circulating current problem of a Modular Multilevel Converter (MMC) leads to power loss and increase in system cost. Based on the analysis of the mechanism of an MMC circulating current, the control principles of three kinds of circulation suppressors based on two-frequency negative sequence coordinate transformation, quasi-proportional resonance and internal mode control are described. An index to evaluate the performance of the circulating current suppression is proposed, namely the ratio of circulation current harmonics. This paper compares the index values of three strategies under three operating conditions, for example steady-state operation, DC voltage step and received active-power step. The control strategy based on internal mode control is most suitable for the steady-state operation of the system. The quasi-proportional resonance control strategy’s anti-interference ability is strong, and is most suitable for the DC voltage step and the received active-power step of the system. This work is supported by the Beijing Natural Science Foundation of China (No. 3172015) and the Science and Technology Project of Beijing Municipal Commission of Education (No. KM201911232016).