The variable frequency constant pressure supply water system based on the expert rule adaptive PID control method was designed according to the ubiquitous characteristics of large inertia, nonlinear, stochastic and pure lag. The structure and composition of the system were introduced in the hardware part. The mathematical model of the system was firstly built in the software part, and then the expert rule adaptive PID control theory was studied, which combines the advantages of high precision of the traditional PID control and strong selfadaption of expert rules. Finally the simulation of various control algorithms was carried on using MATLAB. The simulation results show that the expert rule adaptive PID controller can make the dynamic and steadystate performance optimal, and has a certain application prospect in control systems.