With the rapid increase of wind power intergration, wind turbines such as doubly-fed induction generator (DFIG), are required to participate in active power balance and frequency stability of power system. Therefore, it is necessary to determine the active reserve capacity range of DFIGs. A method is proposed to determine the range of active reserve capacity of DFIGs in overspeed mode. The power flow model of grid-connected DFIGs is established,which takes into account speed and rotor-side converter (RSC) capacity constraints, and determines the lower limit of DFIG steady-state active power. Under this operation mode, the eigenvalue analysis to the power system is performed to find the dangerous modes. Since the slip rate affects the initial value and the state matrix, the Jacobian matrix is extended to establish the analytical expression of the sensitivities of the dangerous mode to slip rate. Based on the above sensitivity, power flow for hazard modes strongly related to slip rate is screened, and adjust PI parameters or/and slip rate are adjusted to eliminate the dangerous mode. Finally, the range of DFIG active reserve capacity is determined based on the slip rate value. The simulation results of the example verify the correctness and control effectiveness of the proposed algorithm.