A tail gate water level control system was developed independently to improve the automatic control of river model testing and ensure the accuracy of experiment. The system was developed by adopting PID regulation algorithm and Lagrange interpolation method. Tests of steady flow and non-steady flow proved that the control system ensured the speed and stability of tail gate level control and improved the accuracy of the experiment. The repetitive precision of tail-water controlling is less than one millimeter. The results suggest that the system could improve the experiment’s repeat accuracy, enhance efficiency, and reduce the cost of experiment.