Subsynchronous control interactions (SSCI) between DFIG-based wind farms and series compensated transmission system have been observed worldwide in recent years, which may lead to generator trip-off and device damage. Many previous studies focus on the suppression of SSCI by flexible AC Transmission System (FACTS) devices. This paper proposes a new static synchronous compensator (STATCOM)-based SSCI mitigation algorithm to increase the stability margin of wind farms. Admittance model of STATCOM with Supplementary Damping Controller (SDC) is derived and it is proved that the controller provides extra damping to the system. Relationships between the controller parameters of the SDC and the damping performance are discussed and the results are validated by simulations with PSCAD/EMTDC. Finally, the impacts of a three-phase fault close to the wind farm is investigated and the resonance can also be alleviated by the SDC under this situation.