Abstract: An automatic weir by waterpower and its work theory of stabilizing the level of water are introduced in this paper. The reliable performance of the weir is validated by an experiment. The precision of the water level stabilized by the device is high,enough to substitute other device used for stabilizing the level of water in lab. This device not only saves the area of the lab and the time for adjusting the water flux;but also increases greatly the precision of experimental simulation. It is of good practical value.