Abstract: For the problem that the climbing ability of existing spherical robots are not strong enough, a new spherical robot mechanism is designed. There are two extendable arms fixed to the spherical body in the direction of body's long axis. When climbing a slope of great gradient, the arms extend and support the body on the road. After arms extension, the robot is driven by motor directly, relying no longer on the pendulum. Then, analysis and comparison of climbing ability before and after arms extension are made, and the dynamic model of linear motion is established when robot climbing, also the path planning after arms extension is analyzed. Finally, the accuracy of the mechanical model is verified by simulation and tests.