The doubly-fed induction generator can provide reactive power to participate in reactive regulation of the grid, but its reactive power range is uncertain. This leads to difficulties in designing voltage control strategies for wind farms. A method to evaluate reactive power range of a wind farm is proposed, and a reactive power control strategy for wind farms participating in grid regulation is given based on the evaluation results. First, the reactive power limit of the turbine is estimated based on the active power. The reactive power capacity composition and calculation method of the wind farm are also analyzed. Then the reactive power capacity of the wind farm is assessed by a two-stage evaluation method. In the first stage, the maximum reactive power support range of the wind farm is obtained, and in the second stage, the reactive power capability is verified considering the uncertainty. A multi-objective optimization problem is constructed with the objectives of reducing voltage deviations, network losses and making full use of the reactive power potential of individual wind turbines. The problem is solved with an optimization algorithm. Finally, the simulation analysis has proved that under the premise of optimum utilization of the reactive power potential of the wind turbines, the voltage deviation and network loss of the wind farm can be reduced. This work is supported by the National Natural Science Foundation of China (No. 51725702).