The control strategy of doubly fed induction generator（DFIG） under grid voltage dips has long been a focus of research and development， grid voltage swells will also cause a lot of problems to DFIG based wind turbines. In order to study the characteristic and control strategy of high voltage ride through（HVRT） of DFIG， this paper analyzes the dynamic behavior of DFIG during grid voltage swells. Based on the above analysis， it is found that the active resistance can effectively suppress the rotor current and torque oscillation， avoiding the operation of Crowbar circuit. As to the different speed and grid voltage swell ratios， this paper proposes variable damping based control strategy， enhancing DFIGs HVRT capability compared with grid voltage dips. Simulation and experimental results verify the effectiveness of the variable damping based control strategy.