The large-scale integration of wind farms and power electronic devices reduces the total inertia and frequency regulation capability of power grid. The new energy systems，including wind power generation system，are required to provide sufficient inertia support to ensure the stability and safety of frequency. The wind power generation system can provide virtual inertia to the power grid by applying the kinetic energy of wind turbine rotor or the electrostatic energy of DC capacitor. However，how to coordinate the two sources to achieve optimal utilization of resources and efficient inertia support is the current research difficulty. On the other hand，the participation of DC capacitor of wind power generation system in virtual inertia provision may lead to sustaining deviation of DC voltage from the rated value. It is difficult for the wind power generation system to cope with subsequent possible disturbances，and the continuous frequency regulation capability of wind power generation system is constrained. Therefore，an improved piecewise coordinated frequency control method for wind power generation system considering voltage restoration is proposed. Firstly，a piecewise coordinated frequency control of wind power generation system considering virtual inertia provision from wind turbine rotor and DC capacitor is constructed. Then，a novel virtual inertia control of DC capacitor considering DC voltage restoration is proposed，based on which an improved piecewise coordinated frequency control of wind power generation system considering voltage restoration is developed. Finally，simulation analysis is carried out in PSCAD/EMTDC. The simulation results show that the proposed control method fulfills the participation of DC capacitor in virtual inertia provision，and it can restore the DC voltage rapidly without affecting the inertia support performance of wind power generation system. The proposed method optimizes the use of frequency regulation resources and improves the grid support capability of wind power generation system under cascading frequency disturbances.