For problems of system driving delay, commutation surge and dithering at a low velocity caused by nonlinearity such as backlash, friction and parameter varieties in a system, a series sliding mode controller for a tank gun control system based on Extended State Observer ( ESO) was designed. Firstly, depending on the measurable signals，a gun control system was decomposed to n first-order subsystems in series, and ESO was introduced to estimate the uncertainty of each subsystem. Then, by adopting a ‘backstepping method’, the sliding mode controllers were designed for each subsystem and made up these sliding mode controllers in series for the gun control system. The examination result shows that the method can restrain the influences caused by the diversified nonlinearity and can improve the system performance effectively.The design of ESO and sliding mode controller is simplified and prove to be realized in engineering because of the design of each first-order subsystem. Furthermore,this method overcomes the dithering problem excisted in traditional systems effectively.