A novel structure of a spherical robot with retractable arms was presented in order to fulfill the requirements of omni-direction movement and operation mission. Under the assumption of rolling without slipping,nonholonomic constraints were revealed and a dynamics model of the proposed robot was constructed by use of Kane’s method. Numerical simulations about rectilinear motion and sigmoid curve motion of the system were carried out in Matlab,and as a comparison,the same trajectories were also implemented by a virtual prototype in ADAMS,which validate the derived dynamical model accordingly. With the derived dynamical model,torques/forces of joints were analyzed. The results indicate the disturbance forces or torques on each joint are not zero under the state of sphere moving,and with rational planning for the trajectory of the robot,there will be a great decrease of the disturbance forces or torques acting on the spherical caps and arms. A novel structure of a spherical robot with retractable arms was presented in order to fulfill the requirements of omni-direction movement and operation mission. Under the assumption of rolling without slipping, nonholonomic constraints were revealed and a dynamics model of the proposed robot was constructed by use of Kane’s method. Numerical simulations about rectilinear motion and sigmoid curve motion of the system were carried out in Matlab, and as a comparison, the same trajectories were also implemented by a virtual prototype in ADAMS, which validate the derived dynamical model accordingly. With the derived dynamical model, torques / forces of joints were analyzed. The results indicate the disturbance forces or torques on each joint are not zero under the state of sphere moving, and with rational planning for the trajectory of the robot, there will be a great decrease of the disturbance forces or torques acting on the spherical caps and arms.