In view of the situation that the workspace of the parallel robot is small,the workspace of an equivalent 8-SPU parallel mechanism is solved and optimized by using the three-dimensional boundary search method. Considering the rod length constraint,angle constraint,link interference,the inverse kinematics equation is established,all points in the workspace is searched,the three-dimensional boundary diagram with fixed posture is given,and then the influential curve of two mechanism parameters on workspace is given. Finally,the ratio of the volume with stroke of the rod is taken as the objective function,and particle swarm optimization algorithm is used to get the maximum function value. The global optimal solution is obtained,and the optimized three-dimensional diagram and the comprehensive indices comparison between unoptimized and optimized is given. The results show that the volume of workspace increases 143. 91% with 27. 59% raise of the stroke and the optimization effect is obvious.