Dynamic analysis and performance optimization of the threetranslation parallel mechanism（PM）2P4R(Pa)-PR(Pa)R proposed by the authors were performed. Firstly, based on the symbolic positive solution of the mechanism, the (angular) velocity and (angular) acceleration of each member were obtained directly. Then, the dynamic model of the mechanism was established by using the sequential single-open-chain (SOC) method based on the virtual work principle. The SOC was taken as the basic unit, and the coefficient matrix of the dynamic equation can be directly called by the results of the mechanism velocity analysis, which improved the calculation efficiency. The correctness of the dynamic modeling was verified by comparing the theoretical calculation values of the three driving forces with the simulation results. Further, based on the differential evolution algorithm, taking the length and mass parameters of the rod as the design variables and the two performance indexes of reachable workspace and energy transfer efficiency as the objective functions, the two-level progressive optimization of the kinematic and dynamic performance of the mechanism was carried out, and the optimal solution of the mechanism under the above two performance indexes was obtained, which laid a foundation for the design of mechanism and the development of prototype. The three-translation parallel mechanism not only had analytic positive solution, but also had partial input-output motion decoupling, which was conducive to the trajectory planning and motion control of the mechanism. The mechanism was suitable for handling, grasping, loading and unloading of large-size workpieces along the length direction.