With more and more wind-turbine generators being integrated into power system， asynchronous wind turbine generators may cause subsynchronous oscillations in the power system with series compensated transmission line. Unified power flow controller（UPFC） can maintain system stability and increase the subsynchronous damping to mitigate subsynchronous oscillations. In order to achieve required subsynchronous damping torque， the control strategy for UPFC is improved and additional control is introduced into the pitch angle control. A test system is constructed by embedding wind farm and UPFC model into IEEE first benchmark model. The results validate the proposed strategy.