A parallel mechanism was applied to the mechanical leg of a six-legged robot to expand the application areas of the six-legged robot. A new mechanical leg based on the (U+UPR)P+UPS parallel mechanism was proposed, and the structure parameters of the mechanical leg were designed. Firstly, the kinematics of leg mechanism was discussed, and the inverse position and velocity mapping equations were presented. Then, the workspace of the leg mechanism was discussed and a 3D map of the workspace was drawn. The performance evaluation index of workspace was defined and the relationship curve of the structure parameters and the evaluation index of workspace was given. Furthermore, the kinematics dexterity of the leg mechanism was analyzed, and the 3D map for condition numbers of Jacobian matrix was drawn. The performance evaluation index of kinematics dexterity was defined, and the relationship curve of the structure parameters and the index of kinematics dexterity were given. Finally, based on the indexes of workspace and kinematic dexterity, the structure parameters were analyzed by Monte Carlo method. A set of excellent structural parameters were proposed, and a 3-DOF mechanical leg was designed. These results lay the theoretical foundation for the further study of six-legged robots.