Abstract: This paper presents the triaxial test results of purple argillaceous siltstone of the Badong formation, which is encountered in the Yichang to Badong Highway. It is found based on the study of stress-strain full procedure curves that the essence of the destruction of the strength of rock is the continual emergency, expansion, accumulation and degradation of native joints. The characteristics of the initial damage are discussed in this paper and the results show that there is a liner relationship between the logarithmic of principal stress difference and the confining pressure. The infinitesimal strength of rock is assumed to obey the Weibull random distribution. The parameter of the infinitesimal strength of rock is reasonably derived and the constitutive model of damage and softening which reflect the whole process of rock failure is established. The parabolic mathematical relationship between the damage variable and the principal stress difference is also analyzed based on the Weibull random distribution. Finally, a conclusion is drawn that the constitutive model of damage and softening is reasonable through the comparison between the test data and the model data.