The four-tank system is a nonlinear multiple-input multiple-output system with strong coupling characteristics. Eliminating or weakening the strong coupling between variables is an effective way to improve the control performance of the four-tank system. Based on the double closed-loop structure, the coupling effect generated inside the water tank is first included in the total disturbance as internal interference, an extended state observer is introduced to estimate it, and the linear state error feedback control law is designed to compensate for the tracking error to form self-decoupling control (SDC). The simulation results show that the tracking performance, anti-interference performance and decoupling of the proposed method have achieved satisfactory results.