In order to enhance low-voltage ride through (LVRT) ability of wind power system with doubly-fed induction generator (DFIG) under grid faults, a forced excitation control strategy for DFIG rotor side converter (RSC) is proposed. A forced excitation multi-frequency proportional resonant (MFPR) controller is added to stator flux vector based control strategy. In case of stator voltage dips caused by grid faults, forced excitation resonant controller is able to compensate output voltage of the rotor side converter. Fault current in the rotor is suppressed and LVRT for DFIG is realized. Also, the relationship between rotor voltage class and LVRT region of DFIG is analyzed, providing design reference of voltage class for DFIG rotor side converter. The control system is simple and has fast response. It can be used to suppress fault current induced by symmetric and asymmetric voltage dips. Simulations on 1.5 MW wind power system are performed to verify the correctness of theoretical analysis and the feasibility of the proposed control strategy.