Abstract: With the continuous development of flexible DC transmission engineering, stability issues have gradually become prominent, and multiple flexible DC projects at home and abroad have experienced low-frequency oscillation accidents on the DC side. A composite virtual impedance controller is proposed for suppressing DC side oscillation in modular multilevel converter(MMC) DC transmission systems. Compared to traditional virtual impedance controllers, this strategy can effectively improve the damping characteristics of MMC and significantly enhance the steady-state performance of MMC back-to-back DC transmission systems at oscillation frequencies. Firstly, a multi-harmonic linearization method is used to establish an impedance model for the DC side of the converter considering frequency coupling effects. The model took into account circulating current suppressors and inner and outer loop controllers, and is extended to impedance modeling of dual ended MMC-HVDC systems. Secondly, impedance analysis method is used to conduct mechanism research on stability issues, and a suppression strategy for composite virtual impedance controller is proposed. The controller characteristics and theoretical effectiveness of traditional virtual impedance and the proposed strategy are compared. Finally, the MMC back-to-back DC transmission system is built in Matlab/Simulink, and the accuracy of impedance modeling and the effectiveness of the composite virtual impedance controller are verified. Furthermore, the influence of the parameters of the composite virtual impedance controller on the dynamic performance of the system is analyzed.