Abstract: Applies discrete-time variable structure control to the design of balance controller for Acrobot system. Develops a discrete-time mathematical model for Acrobot system at its upright equilibrium point to analyse the mechanism of chattering which occurs in compliance with the discrete-time exponential approximation law in a variable structure control systems. To improve the discrete-time exponential approximation law, a modified one is presented and applied to the design of discrete-time variable structure balance controller for Acrobot. Simulation results showed that the modified exponential approximation law will not only take the advantage of original one, but also reduce the chattering effectively, so as to keep the Acrobot system asymptotically stable, thus enabling the discrete-time variable structure control based on sliding mode for Acrobot system to come true.