Taking the spatial three translation parallel mechanism as the research object, according to the design theory and method of azimuth feature set, a kind of spatial pure mobile parallel robot mechanism with motion decoupling, symmetrical structure, simple motion pair and large working space was designed. Through the analysis of topological structure characteristics, it was proved that the mechanism had the motion characteristics of spatial three-dimensional movement, and the kinematic equation model was established and the forward and inverse kinematics solution and Jacobian matrix were derived. Based on this, the singularity, operation space, dexterity and other performance indexes of the mechanism were analyzed.The actual demand of working space maximization was taken as the mathematical optimization model, and the firefly algorithm was selected for optimization design. The results show that the performance of the optimized workspace is significantly improved, and the workspace volume is increased by 12% compared with that before optimization. The optimized parameters provide reference for the application design and control of the mechanism.