Abstract: Modular multilevel converter（MMC） based flexible direct current transmission technology is an important scheme for the power transmission of distant offshore wind farm, but it presents multi-input-multi-output（MIMO） characteristics. The frequency coupling effect of MIMO characteristics poses a new challenge to the stability of the offshore wind farm-MMC interconnection system. Most of the existing MMC models have ignored the frequency coupling effect, and there are few studies on the key mechanism affecting the frequency coupling effect. In view of the above problems, this paper establishes the equivalent sequence impedance model of MMC on the wind field side, including the frequency coupling effect, and analyzes and evaluated the key factors affecting the frequency coupling effect and stability. Meanwhile, an improved MIMO stability criterion based on Gershgorin-circle theorem is proposed to solve the complicated problem of the existing stability criterion, which simplifies the stability judgment process by introducing the distance function. Then, based on the criterion, the influence of the key factors and the frequency coupling effect on the stability of the system is analyzed, and the conservatism of the criterion is analyzed quantitatively. Finally, a simulation example is given to verify the correctness of the theoretical analysis.