Abstract: Firstly, the solid geometrical configuration of the humanoid robot's arm is analyzed, especially the case that constraints on the posture of end-effector are incomplete for the given task. On that basis, rotation angle around the bat's normal direction is optimized to minimize kinetic energy of the entire robot's arm. Meanwhile, the posture of the endeffector is analyzed to make it optimal on the basis of the whole arm's energy optimization and the optimal condition is determined, namely, the plane constructed by the arm is perpendicular to the plane constructed by the velocity vector of the bat's motion. Then, the equations are built on the basis of the optimal condition and the relations between each link, subsequently the optimal end posture is obtained, so that the vibration generated by the robot arm's motion is reduced. Finally, the method's validity is demonstrated by the kinematic simulations from the view of the kinetic energy's solution and the trajectory planning.