Wind farms are mostly located in rural， remote areas where unbalanced grid voltage is a common feature. Unbalanced grid voltage can cause relatively large unbalanced stator and rotor currents， which will further create oscillations in the power and electromagnetic torque in the doubly fed induction generator（DFIG）. The operation conditions of the DFIG system will worsen subsequently. The mechanism of the series grid-side converter（SGSC） to suppress the influence of unbalanced grid voltage on the DFIG system is analyzed. A new control strategy of the DFIG system with SGSC under unbalanced grid voltage conditions is proposed including the control of both parallel grid-side converter（PGSC） and SGSC based on the proportional resonant（P-R） controllers in the stationary reference frame. Not only the double supply frequency oscillations in electromagnetic torque， DC bus voltage and total active power output of the DFIG system can be eliminated， but also the stator and rotor currents of the system are balanced. The rotor-side converter can be still controlled in the conventional manner and a solution to complicated high-order matrix is avoided. Simulations on a 2 MW DFIG system with SGSC verify the validity and efficiency of the proposed control strategy.