Abstract: At present, there is still a lack of systematic research on the medium- & high-frequency resonance mechanism of the modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system. Therefore, based on the partitioning impedance modeling and the harmonic impedance analysis method, this paper systematically reveals the medium- & high-frequency resonance mechanism of an MMC-HVDC transmission system. Firstly, based on the partitioning impedance modeling, the MMC-HVDC transmission system is divided into blocks. Through the analysis of the AC and DC system circuits and the MMC control system, the equivalent impedance models on the MMC AC and DC sides are established respectively. Meanwhile, the impedance model of the transmission line is established according to the line distributed parameter characteristics. Thus, the equivalent model of the MMC-HVDC transmission system is formed. Secondly, by studying the influence of the control links and the parameters on the MMC equivalent impedance, the main influencing factors are obtained. Then the resonance is divided into active resonance and passive resonance, and the resonance mechanism of the MMC-HVDC transmission system is systematically revealed. Finally, the effectiveness and correctness of the proposed model and resonance analysis are verified by simulation. The analysis and simulation results show that only active resonance occurs in the MMC DC-side system at middle- & high-frequency bands, and that the resonance frequency is mainly related to the DC line parameters. Both the passive and the active resonances may occur in the MMC AC-side system, and the passive resonance is mainly affected by the negative damping characteristics of the MMC AC-side impedance, while the active resonance is related to both the MMC and power grid parameters.