Abstract : To improve the control accuracy and reliability of a large-scale variable load constant pressure water supply system, a control strategy of hierarchical control of multiple water pumps and alternating frequency conversion is proposed. Simultaneously, some corresponding improvements are made to the conventional PID (Proportional Integral-Derivative) control algorithm based on the characteristics of the large-scale variable load water supply system. The system hardware, software, and communication are designed through a combination of standard drive products based on the USS protocol and a PLC (Programmable Logic Controller), thereby improving the control quality of the large-scale constant pressure water supply system. Keywords : Fieldbus, USS protocol, PLC, frequency converter speed regulation, interlocking control, PID control. A new and economical constant pressure water supply system composed of a Siemens Microstater Driver frequency converter and a SIMATIC S7-200 PLC has been running in the Changsha Information Building and several other units, with very positive feedback. 1 Overall Design of the Constant Pressure Water Supply System To achieve the purpose of large-scale constant pressure water supply, the water supply system consists of three water pumps and one frequency converter and controller. This approach avoids paralyzing the entire water supply system due to the failure of a single pump. Furthermore, by utilizing the relationship between the pump's operating curve and efficiency, each pump can operate in an energy-efficient and high-performance state in rotation, saving energy and extending pump life. The system's hardware structure is shown in Figure 1. [b][align=center]For more details, please click: Design and Research of Constant Pressure Water Supply System[/align][/b]