The work aims to design a 3T1R parallel robot mechanism to meet the cargo handling requirements of palletizing operation in production line. The topological structure characteristics of the robot mechanism, such as degree of freedom, coupling degree and orientation feature set, were analyzed by the position and orientation characteristics equation. According to the configuration of the kinematic pair, the inverse kinematics model was established. Considering the accuracy requirement of the motion range of the palletizing robot, a novel numerical search method was adopted to realize the fast and efficient search of the workspace of the parallel mechanism. At the same time, the influence of the structural parameters on the workspace volume was analyzed, and the whale optimization algorithm was selected to optimize the structural parameters.The workspace obtained through the analysis was large and continuous. The novel numerical search method could quickly and effectively search the exact workspace boundary. The optimized parallel mechanism has good workspace and promising application prospect.