Abstract: The increasing penetration of renewable energy in the power system leads to the continuous decline of system inertia, weakening the system frequency stability. As a representative of renewable energy, grid-forming controlled wind power is an effective method to improve the frequency stability level of the power system. However, the current control methods for virtual synchronous generator (VSG) based wind turbines do not sufficiently consider the dynamic characteristics of the machine side on the wind turbine involved in the frequency support process, which creates a risk of rotor instability due to rotor speed stalling. Because of the above problems, this paper first analyzes the frequency response characteristics of grid-forming controlled type-Ⅳ wind turbines. Then, the coupling relationship between the power angle and rotor speed is analyzed, and the characteristics of rotor instability and the influence of control parameters are clarified. Based on the above analysis, a VSG-based grid-forming frequency support control for type-Ⅳ wind turbines considering the rotor stability constraint is proposed, which improves the system frequency stability under various scales of disturbances and effectively avoids the secondary frequency drop during rotor speed recovery of wind turbines. Finally, a case study is simulated in the PSCAD/EMTDC to verify the effectiveness of the proposed strategy.