Abstract: Variable-pressure water supply is considered to be a good operation mode to save energy. But lacking enough study on the control model results in pressure fluctuation at user side, which will restrict variable-pressure water supply for application and popularization. To improve the control quality, a step response experiment rig of water-supply system was set up based on virtual instrument and frequency converter. A LabVIEW test program was compiled to carry out the step response experiment at different openings of valve. And then process models were constructed with system identification toolbox of Matlab according to experiment data. Contrasting the theoretical and experimental models, variations among different processes were analyzed from vertical and horizontal aspects. The results showed that the variable-pressure operation had a similar variation trend in structure and parameters, and it also had more obvious inertia, time varying and hysteretic nature. So the transient state performance of the variable-pressure water supply system should be considered for design of variable-parameter controller with PID.