To deal with the problems of safe thickness at the end of the pile in Karst areas, the Hoek-Brown rock mass strength criterion was introduced, which considered the influence of rock mass quality and triaxial stress. The formula of rock mass tensile strength was confirmed，and a practical formula of rock mass shear strength was established by the use of Lambe transformation. Then, the computational method of the safe thickness of the rock mass at the end of the pile was determined on the basis of the failure mode of the rock mass at the end of the pile. Parametric analysis has shown that the Rock Mass Raling(RMR) value has a strong effect on safe thickness. The higher the RMR value, the smaller the safe thickness and the greater the contrast. For the general state of the natural homeostasis of the Karst cave roof, a safe thickness value 3 times the pile diameter is reasonable. Finally, a sample project was carried out to verify this calculation method.