The conventional decoupling control in the variable-speed doubly-fed wind turbines has little support to the system frequency. The characteristics of doubly-fed induction generator wind turbines in the system frequency control are analyzed. A revised frequency control based on the conventional decoupling control is proposed. The control system consists of four functional modules， i.e. frequency control， rotational speed delaying recovery， speed protection and coordination control with conventional generators. Simulation results show that the control strategy has a fast response speed to the transient frequency error. The rotor speed can also reach to the best operation status quickly. It proves that wind farms can participate in the system frequency regulation to a certain extent.