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Aiming at the problem of kinematic performance optimization of three degree of freedom (3-DOF) two rotations and one translations parallel mechanism driven by outer pair, a multi-objective optimization design method of the dimension synthesis was proposed. The Screw theory was used to analyze the kinematics performance and obtain the inverse solution of the position for the parallel mechanism. The reachable workspace was divided into n layers, the polar coordinate method was used to calculate the maximum inscribed circle at each layer, and the numerical method was adopted to calculate the volume of the regular workspace of the frustum of a cone formed by the maximum inscribed circle of each layer. Considering the volume of the regular workspace and global motion/force transmission index as the objective functions, driving range and rotation angle of joints as the constraint conditions, and the mechanism parameters as the design variables, the multi-objective particle swarm optimization algorithm was used to find the Pareto optimal front of the objective functions, the results showed that the two objective functions conflicted with each other. The optimal value of GTI at the front of Pareto was increased by 57.57% compared with that before optimization, and the optimal value of Vr was increased by 37.59% compared with that before optimization. The comparison results before and after optimization showed effectiveness of the optimization algorithm.