PID controller can not perform well in liquid level control system because liquid system has characteristics of time-delay and inertia. Based on the characteristics of two-tank liquid control system, an ADRC is designed by using control technique of active disturbance rejection. In order to study the control performance of ADRC, the comparative analysis with PID controller is to be carried out in the research. Simulation research shows that water level control system based on ADRC possesses better rapidity, less overshoot and strong disturbance inhibition compared with PID controller. Thus, the research result is of important guidance for engineering of water level control.