Abstract: To solve the problem of sub-synchronous oscillation caused by the interaction of sub-module structures in modular multilevel converters, which affects the stable operation of the system, a control structure is proposed, which considers the combination of the voltage of the sub module capacitor and the suppression of cyclic current. By establishing a more accurate DC impedance model of the modular multi-stage converter, the second-order cyclic current of the sub-module is reduced. And Nyquist criterion was used to analyze system stability and voltage and current changes under different controller adjustments. The results show that the new model has a better fit with the internal structure of the actual modular multi-stage converter, and can accurately predict the stability of the DC system. This verifies the effectiveness of the circulating current suppressor combined with the voltage control of the sub module capacitor.