Abstract: In order to increase the workspace and the carrying capacity of robot shoulder joint, a robot shoulder joint was proposed, which based on three degrees of freedom orthogonal spherical parallel mechanism. In the article, the dynamics model of the shoulder joint was established using the second lagrange method. After derivation, expressions of inertia moment, coriolis force-centrifugal force, and gravity were obtained. Based on the above dynamic model, the peak prediction model of servo motor for the shoulder joint was defined. Further more, by the trajectory of the moving platform, the change regularity was analyzed that caused by the shoulder joint servo motor driven angular velocity and driving torque, and received dynamics of shoulder joint. Then, the peak prediction model of servo motor for the shoulder joint was validated, and obtained that maximum estimated peak torque was 4.27 N·m. The results show that the angular shoulder and drive torque of shoulder display periodic variation. The peak prediction model of servo motor can provide theoretical base for the servo motor selection of the shoulder joint.