Abstract: To increase the anti-disturbance ability of Acrobot systems, a dynamic model for the underactuated Acrobot system was built, and a silding mode control strategy for the system was proposed. In the strategy, the model in cascade normal form is expanded to the common form with disturbance term, and the hyperolic tangent function is used to substitute the sign function to eliminate chattering. The Acobot system with the proposed control strategy was proved asymptotically stable and robust using Lyapunov theory and perturbation theory. This method can be applied to other underactuated systems. Simulation results show the effectiveness of the proposed control strategy to Acrobot balance control.