In this paper, the battery energy storage supplementary damping control is introduced to suppress the subsynchronous oscillation of DFIG-based wind farm interfaced with series compensation. Firstly, the conventional control strategy of battery energy storage system is analyzed, and the small signal model of doubly-fed wind farm with battery energy storage is established. Then the eigenvalue method is used to analyze the subsynchronous oscillation mode and related participation factors. Based on the above analysis, the supplementary damping control strategy of battery energy storage is designed to suppress the subsynchronous oscillation of DFIG-based wind farm interfaced with series compensation. Finally, the PSCAD electromagnetic transient simulation model is established. The time domain simulation method is used to verify the effectiveness of the proposed control strategy.