For the geotechnical engineering, rock is usually subjected to spatial stress state, so it is rather meaningful to study the mechanism of rock damage and fracture under triaxial stress contidion.The rock is heterogeneous material and its meso-mechanical property satisfies the Weibull random distribution. Based on random distribution of interior defect in rock, a statistical damage constitutive model for rock was developed by using the statistical theory subjected to confining pressure and external force. The statistical parameters in the constitutive model were solved by using triaxial compression of sandstone, and qualitativly analyzed the influence of confining pressure to critical damage. The results show: （1）The model can reflect the confining pressure effect and the variation law of the rock strength with the confining pressure; The way of determining the parameters indicated the physical meaning of the parameters of model;（2）The statistical parameters in the constitutive model were solved by using the characteristics of the full stress-strain curve of sandstone acting on the specific confining pressure. Different parameters were corresponding to different confining pressure;（3）The crical damge changed with the confining pressure and depended on the state of stress due to the loads acting on rock; (4) According to physical meaning for the distributed parameters m, m can judge the brittleness and 1/m can judge the ductileness for sandstone.
