Giving an example of a small second pressing water works, the principle, construction and characteristics as well as the application of a automatic constant pressure closed-loop water supply and monitor system, composed of PLC and industrial control configuration software and frequency converter, is introduced in this paper. The system, which is tested in the applying scene, has the features of operating simple, moving reliable, automatic level high, protecting function completely and saving-energy better. It has the marked economic and social benefit as well as the value of spreading application also.