To mitigate the sub-synchronous control interaction (SSCI) between a doubly-fed induction generator (DFIG)-based wind farm and a series compensated transmission line, an improved passive control strategy to mitigate the SSCI is proposed. First, the mechanism of the SSCI in the DFIG-based wind farm is analyzed. It is found that the double-loop PI controller of the rotor side converter has notable impact on the SSCI, and hence it may be improved to mitigate SSCI. Second, the passive controller to the rotor side converter is designed based on the Euler-Lagrange (EL) model and the passive theory. This controller mitigates SSCI by the finding the steady state point and injecting the damping. To enhance the mitigation capability, a band pass filter is introduced to improve passive control. The input signal is maintained stable to enhance the controller performance. Finally, numerical results using eigen-analysis and time domain analysis are compared with those using the PI control and the normal passive control. The results validate the mitigation effect of the proposed strategy at different compensation levels and wind speeds.