Abstract: Geological conditions of strata, properties of rocks, and results of classification of rock masses at Chuankou Wolfram mine are introduced briefly. Bases on the in-situ investigation and survey in underground, the comparison sketch of boundary of underground excavating opening and the collapse border of surface is presented; and caving characteristics is analyzed and a typical collapse of straight part of covering strata is proved. According to the different ratings of joint-fissure between country rockmass of opening and covering strata, Terzaghi's theory is used to explain the mechanism and fracturing characteristics of country rockmass surrounding the opening and covering strata. Then Terzaghi's theory is improved according to the properties of rockmasses, and pressure formula is derived ; for the more, limited equilibrium method is employed for analyzing the stability of covering strata with different mining spans. On condition that the collapse does not occur, the quantitative relationships between safe mining spans and coefficients with different heights of covering strata.