When the pressure of suction side of the pump is too low and even less than the saturated pressure that the water temperature is corresponding to, one pat of water will vaporize to form water bubbles, and when these bubbles are taken by water to the position with higher pressure along the impeller, they will congeal suddenly, therefore they burst. The water along the bubbles quickly fills up the space occupied by the bubbles to produce hydraulic impact, namely water function, which continuously impacts the metal surface to make the water pump generate noise, vibration and low efficiency, thus the impeller is corroded and damaged, in severe circumstances, the water pump is forced to stop.