In recent years, the impedance modeling of offshore wind power through flexible direct transmission system has been extensively studied, but the influence of impedance coupling is often ignored. Meanwhile, the rise of grid-forming (GFM) control makes some power electronic equipment change the control structure, and there is not enough research on the interaction stability between GFM control converter and modular multilevel converters (MMC). Therefore, this article proposes an impedance modeling method based harmonic state space for GFM control voltage source converters (VSC) and MMC. Firstly, the offshore wind power transmission system is described and analyzed. Then, the impedance models of MMC and VSC are proposed respectively, and the impedance model of single-input single-output system is obtained through the signal flow diagram. Finally, a simulation system is built to verify the influence of impedance coupling on the MMC and VSC impedance, and the stability analysis is carried out by changing the control parameters.