We build mechanics models that are based on the parameter of mining technology conditionsand rock mechanics property in a mine and use RFPAZ0 numerical simulation software to forecast theheight of overlying rock’waterflowing fractured zone and use empirical formula to estimate theheight and measure actually the height of cavity’working face.We compare the numerical simula-tion’s height using software and the actual measurement’s height,the result demonstratesthat they are mostly equal.So the numerical simulation’s result can be the basis to carry out the minewater prention and control.but we must consider the forecast deviation ratio.