In this paper, the author researches on the design of constant pressure water supply system based on PLC. The combination of computer technology and PLC control technology could effectively and rapidly test the present water supply system’s parameters and performance; meanwhile it could also modify the software to test other parameters at any time according to customer’s demands. This study adopted modular design, usually used in system design, to divide the system’s functions into modules, thus revealing the design concepts of the complex system. The control model and the approximate mathematical model are also established. The frequency converter with built-in PID module to realize the closed loop speed control system. The water flow with frequency pump mutual investment security supply and variable frequency pump grading adjusting water supply mode can not only ensure the lowest safe water flow and also high flow constant pressure water supply and energy saving. This paper discusses the two motor mutual investment reserves for minimum security of water supply idea and implementation method, discusses a variety of effective supply states and the switching conditions of four pumps parallel water supply mode. This paper also discusses the PLC control software program flow, electrical control scheme, PLC software, PID parameter setting, motor relay protection setting and some other issues in depth.