Because of complex size parameters and dynamic performance in high-speed 6—PSS parallel robot, a multi-objective optimization method based on work space was proposed. The aim of this method was to improve the dynamic performance. At first, the kinematic equations of parallel robot were calculated, the dynamics equation was solved by using the Lagrange equations, and the optimize range of size parameters was obtained according to the constraints analysis and work space analysis of parallel robot. Then, the influence trend of various parameters on the critical dynamic performance was analyzed. The optimization objective function was proposed by using comprehensive analysis of the results. At last, the size parameter optimal solution of parallel robot was obtained by using optimized mathematical model. Through the simulation, the dynamic performance values before and after optimization, including the driving force, speed, power, etc. were compared which could prove the correctness of the proposed optimization method. The obtained results showed that the optimization algorithm was correct and effective, which can provide an effective method for optimizing parameters design in high-speed parallel mechanism.