The devices of flexible AC transmission system（FACTS） are applied in power systems increasingly. This may result in subsynchronous oscillations. Intensive oscillation will lead to serious damage on generator rotor shaft， which poses a high risk on power system security. Supplementary impedance control method of thyristor controlled series compensation（TCSC）， which is based on local measured impedance lifting coefficients， is used to damp subsynchronous oscillations. The effectiveness of this method is verified by a simulation example， which includes thermal power units in power system with TCSC. The simulation results show that this method is as effective as the method based on the angular speed deviation. As it is simple and easy to be realized， the method is suitable for engineering application in power system.