Abstract: The inclination angle of the failure plane of rock mass is the angle between the fracture surface and the maximum principal stress, it is an important parameter in the design of the rock engineering projects, and serves as an important basis for the security warning in engineering projects. The inclination angle of the failure surface of rock mass is influenced by the stress, fabric, water and other factors, giving rise to great uncertainty in the determination of the inclination angle of the failure surface, especially for weak rock mass. The inclination angle of failure surface is 45°+φ/2 as given by the Mohr-Coulomb failure criterion and it is a constant. In fact, the inclination angle of failure surface is found to fall in a range instead of being constant, so it is necessary to carry out a study on the inclination angle of failure surface. By focusing on the failure of the silty mudstones under the hydraulic-mechanical coupling condition, which is a typical soft rock in the red beds in South China, this paper has derived a mathematical expression of the inclination angle of failure surface of rock mass, with the use of the probability distribution function of micro cracks in the rock mass. Three axial compression tests have been done under the hydraulic-mechanical coupling condition in the TAW-100(hydro-mechanical coupling and mciro-mechanical triaxial test equipment with servo-control loading system).According to the derived mathematical expression, the range of the inclination angle of failure surface is determined to be 50.3°~80.2° with the confining pressure being 0 and 1MPa. Furthermore current results are compared with the prediction of the Mohr-Coulomb theory, as well as the experimental results. It is found that the results obtained by the method of probability is close to the experimental results, suggesting that this method is reasonable and reliable.