In order to solve the problem that the management of water-supply operation is still backward and the phenomenon of water resources wasting is exceedingly serious, this paper introduce some advanced technologies such as computer, frequency transformer, programmable logical controller (PLC) and so on into water-supply network, the constant pressure water supply system is devised to achieve the variable-frequency speed control by making use of the electric controlling system composed of S7-200PLC, MM440 frequency transformer, miniature circuit breaker, AC contactor, thermal relay, DC power supply, miniature electro-magnetic relay, sorts of indicator lights and host command devices.