This Aiming at the characteristic that the three-capacity water tank system is a typical non-linear and large lag object and has a strong representativeness in simulating industrial process control, a liquid level control method based on expert PID is proposed, and the control is designed according to the knowledge base of the expert system. According to the rules, real-time correction of PID controller parameters improves the precise control of the water tank level. Modeling and simulation in MATLAB and comparison with conventional PID control show that the expert PID algorithm has dynamic characteristics such as small overshoot and short settling time.