The supplementary excitation damping controller（SEDC） of the turbine-generator has been proven to be one of the most economical and convenient methods for damping of subsynchronous resonance（SSR） in the power system with the fixed series compensation. A novel SEDC model is presented based on independent modal space control theory. By this means， gain and phase compensator can be designed separately without interaction. An improved particle swarm optimization（PSO） algorithm for SEDC parameters design is proposed. To improve this algorithm， the mutation mechanism is involved to improve the global convergence capability and to escape from the locally optimal solutions. A case study with practical background is employed to validate the proposed algorithm. The eigenvalue analysis and time domain simulation methods are used. The results show that the SEDC designed by improved PSO algorithm can damp multi-mode SSR oscillations effectively and robustly.