Aiming at the sub-synchronous oscillation in the direct-drive permanent magnet synchronous wind turbine generator, a complete mathematical model of direct-drive wind turbine with introducing the sub-synchronous oscillation damping controller (SSDC) is established. Based on it, a multi-objective optimization model of SSDC parameters is established, which takes both real part of oscillation mode eigenvalue and damping ratio as objective functions. In order to solve the problem of poor population diversity in the process of parameters optimization of improved nondominated sorting genetic algorithm (NSGA-II), selection strategy of NSGA-II algorithm is perfected, and the improved NSGA-II algorithm is introduced into the design of SSDC parameters optimization, thus evaluating solution set by space evaluation method, the results show that Pareto optimal distribution obtained by the improved algorithm is more uniform. Finally, through eigenvalue analysis and simulation verification, it confirms that the SSDC optimized in this paper is of great significance to inhibit SSO in the direct-drive wind turbine.