To solve the problem of “creeping” and oscillation when the tank gun controlling system operates at a low speed, a strategy of sliding-mode variable structure control based on the disturbance observer is proposed. The double closed-loop mathematical model of the current and rotary speed of the tank gun-controlling system is studied, and the sliding-mode variable structure controller is designed. A disturbance observer is designed to observe the time-varying load torque and friction torque in the control law. The simulation results show that the control strategy can not only solve the problem of “creeping” and oscillation when the tank gun controlling system operates at a low speed, but also weaken the chattering of the sliding mode caused by the modeling uncertainty.