Aiming at the problem of insufficient frequency modulation capability of direct-driven wind turbine under traditional virtual inertia control，a control strategy of super capacitor assists direct-driven wind turbine participating in frequency modulation was proposed to improve the frequency stability of power grid after wind power is connected. Firstly，the influence of the fan speed and output power on the frequency modulation capability when the direct-driven wind turbine participates in the frequency modulation through virtual inertia control is analyzed，and the frequency modulation coefficient was established through the Sigmoid function to characterize the frequency modulation capability of it under different operating conditions. Then，based on the frequency modulation coefficient，a frequency modulation strategy for wind storage was proposed，that is when the frequency modulation ability of direct-driven wind turbine is strong，the direct-driven wind turbine participates in the frequency modulation independently，while when the frequency modulation capability is weak，the power required for the frequency modulation is jointly provided by the direct-driven wind turbine and the super capacitor. At the same time，the charging and discharging coefficient was introduced into the power control of the super capacitor to avoid its over-charging and over-discharging. At last，the simulation results show that the proposed method is better than the traditional virtual inertia control when frequency modulation capability of the direct-driven wind turbine is insufficient，and it also avoids overcharging and discharging of super capacitor.