Abstract: In order to suppress the sub-synchronous oscillation in DFIGs series compensation system,the mode analysis shows that the dominant sub-synchronous oscillation mode is strongly related to the stator and rotor currents. Considering the existence of flexible controllable converters on the rotor side,a damping reactor is proposed to be added to the rotor side,and a non-linear damper controller is designed to suppress the rotor current fluctuation（disturbance）by using partial feedback linearization method in order to suppress the subsynchronous oscillation of the system. The realization block diagram and parameter configuration principle of integrating nonlinear damping controller into RSC are given. By means of small disturbance analysis（frequency sweep,electric damping and mode analysis）,the performance of nonlinear damping controller in suppressing subsynchronous oscillation is analyzed. It is verified by the time domain simulation when the subsynchronous oscillation is in instable and low damping state.