This paper uses the self-decoupling control (SDC) method to study the liquid level control of a three-tank system. Firstly, a linear extended state observer is employed to estimate and compensate for the coupling, non-linear elements, and disturbances in the three-tank system. Subsequently, by designing appropriate control laws, the self-decoupling of each subsystem of the system is achieved. Simulation results indicate that compared with traditional active disturbance rejection control, the self-decoupling control method demonstrates great dynamic tracking performance for the desired liquid level values with faster response speed and better robustness.