Aiming at the problems that the existing impedance modeling of virtual synchronous generator (VSG) based modular multilevel converter (MMC) is not sufficiently accurate and fails to systematically reveal the influence of control parameters on system characteristics in the VSC-HVDC transmission scenario, this paper conducts research on impedance modeling and stability characteristics of VSG-MMC. Firstly, based on the multi-frequency steady-state harmonic characteristics of MMC, a complete sequence impedance model of the AC side of VSG-MMC including voltage disturbance small-signal and excitation module is established, and the differentiated regulation paths of three core parameters, namely virtual inertia, virtual damping and reactive power droop integral coefficient, on the amplitude-frequency and phase-frequency characteristics of impedance are clarified. Secondly, on the basis of establishing the power small-signal model of VSG-MMC, the influence of parameter variation on the system eigenvalue distribution and dynamic performance is quantified. Finally, a simulation test platform is built in Simulink to verify the reliability of the model, compare the stability of the VSG-MMC system under different short-circuit ratios, investigate the regulation law of VSG control parameters on the AC-side impedance characteristics of MMC, and provide theoretical basis and reference for the control parameter design of VSG-MMC combined with the influence of parameter variation on system dynamic performance.