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The square mean dexterity coefficient, which is used to comprehensively evaluate the dexterity of various configurations, is introduced. The influence law of the structural parameters, including the distribution position angle of the joints on the stationary platform and moving platform, the distribution position radius of the joints on the stationary platform, and the distribution position radius of the joints on the moving platform, on the square mean dexterity coefficient are discussed. On these grounds, the structural parameters are optimized. The optimal results obtained are as following, the distribution position angle of the joints on the stationary platform is π/2, the distribution position angle of the joints on the moving platform is 2π/5, the distance between the first joint on the stationary platform and the center of the stationary platform is 650 mm, the distribution position radius of the joints on the stationary platform is 800 mm, and the distribution position radius of the joints on the moving platform is 160 mm. The simulation results show that the square mean dexterity coefficient of 4-UPS-RPS parallel mechanism are creased, so that the obvious improvement of the kinematics characteristics for the parallel mechanism is realized.