Abstract: The back-to-back system based on modular multilevel converter (MMC-BTB) has close control interaction between converters, which leads to frequent oscillation accidents. In the existing research, the single-station model ignoring harmonic transmission cannot accurately describe the impedance characteristics of MMC-BTB, and the coordination between the two converters is less frequently considered from the perspective of oscillation suppression. Therefore, this paper considers the mechanism of harmonic transmission and attenuation between converters in MMC-BTB, and proposes an oscillation suppression strategy based on the impedance equivalence of various circulation control methods. First, the flow path of harmonic is qualitatively analyzed based on the equivalent circuit, and the double-station model of MMC-BTB is derived quantitatively. Then, the interaction mechanism of the circulation control on both stations along the harmonic path is analyzed, which demonstrates its key role in improving the impedance characteristics of MMC-BTB, and proposes a strategy for coordination and parameter optimization of circulation control. Finally, the correctness of the theoretical analysis and the proposed strategy is verified through Matlab/Simulink.