Abstract: The complex power electronic features of the modular multilevel converter (MMC) result in the intricate multi-frequency coupling with AC and DC power grids. Based on the principle of harmonic state space, a small signal model of the MMC main circuit and the control system including the vector current controller and the circulating current suppression controller is developed in this paper. The MMC system model is firstly integrated through the interface relationship between the main circuit and the control system, which covers the dynamic characteristics of multi-frequency harmonics. Then, based on this model, the calculation method of the harmonic transfer function is proposed. The global relationship of the harmonic coupling impedance of the AC, DC current and the internal circulation of the MMC is given according to the harmonic transfer function matrix, and the coupling characteristics of the disturbing harmonics and the three different current harmonic components are further analyzed. Finally, the comparison between the simulated and calculated values of the electromagnetic transient simulation model and the developed small-signal model under different conditions verifies the validity of the small-signal modeling and the correctness of the harmonic coupling analysis.