Due to the weak coupling between variable speed wind turbines（VSWTs）and synchronized main networks，the system equivalent rotation inertia and primary frequency control ability are decreased with the continuously increasing wind power penetration.To solve the problems，VSWT with additional frequency control is studied.The dynamic characteristics of input mechanical power and output active power of VSWT during the frequency response process are analyzed，and the relationship between the active power increment and the duration of VSWT participating in the system frequency regulation is quantified.A coordinated frequency control strategy of wind farm is proposed.According to the control strategy，the VSWTs can participate in frequency regulation depending on the coordination of wind speed，power increment and duration.Simulation results demonstrate the effectiveness of the proposed control strategy.It not only makes full use of the frequency regulation ability of VSWTs but also avoids the negative effects participating in the system frequency regulation.