Abstract: A novel 5UPS-RPS 5-dof parallel mechanism with one RPS-type constraint leg and five of the same UPS-type active legs was presented.First,the characteristics of this PM were introduced,the first-order and second-order comprehensive influence coefficient matrix were derived by virtual mechanism method,and analytic formulas for solving the velocity and acceleration of this PM were derived systematically.Second,the velocity global performance index for the optimization of this parallel mechanism in the workspace was derived based on the condition number of Jacobian matrix. Then the dimensions of the fixed base and moving platform with good kinematic performances were derived in the workspace. Furthermore, theoretical analysis on velocity global performance index provides a foundation for the optimization design of this parallel mechanism in the future work.