The SVC-based subsynchronous damping controller（SSDC） is optimized to damp multimodal subsynchronous resonance（SSR） in the Jinjie（power plant） series-compensated transmission system. Firstly， linearized models of the multi-machine system including the SVC are obtained. Then， a control scheme is proposed based on the principle of independent model control. Next， SSDC parameter tuning is formulated into a constrained nonlinear optimization problem. The GASA is implemented to solve the problem and obtain the optimal parameters. Finally， eigenvalue analysis and electromagnetic simulation are conducted to verify the effectiveness of the developed controller. The results demonstrate that SVC-SSDC can greatly improve the damping of the three torsional modes and thus effectively depress the multimodal SSR， ensuring the stability of the system and the safety of the generator shafts.