The subsynchronous oscillation of the power system can be effectively suppressed by the supplementary subsynchronous damping controller (SSDC). Parameter design processes of phase compensator and proportional gain are the key parts of the SSDC parameter design. Firstly, the design method of the parameters of the phase compensator is proposed in this paper. And based on the determination of the compensation phase of the SSDC, the design method of the paramenter of the proportional gain is proposed. Finally, the correctness of the proposed paramenter design method of SSDC is verified by case study.