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In order to investigate the constant pressure control characteristics of centrifugal pump, a testing system on constant pressure closed-loop control of centrifugal pump water-supply system,，which was based on internal PID controller of frequency converter was set up by using the virtual instrument. An innovative form of the outlet of the pump system composed of three ball valves was remodeled. It could change working condition quantitatively and rapidly. The outlet could create similar step disturbance. A LabVIEW test program was compiled to do research on control characteristics of constant pressure centrifugal pump system. The set range of the given pressure of the constant pressure control system was concluded. Contrasting the control characteristics of the fixed parameter PID regulator under different working conditions, it is showed that the control characteristics of the fixed parameter PID regulator are not good under the working conditions that changes quickly and extensively. Analyzing the control characteristics of the fixed parameter PID regulator and the variable parameter PID regulator under the same working condition, it is discovered that the variable parameter PID regulator is more applicable for the constant pressure control system.