Variability in the physical properties of rock mass is a major source of uncertainty encountered in railway tunnel stability analysis. The conventional safety factor method on stability assessment of railway tunnel within soft and weak rock mass can not represent the variability of rock mass. A formulation to compute the reliability of soft and weak rock mass, in which physical non-linearity is taken into account, is proposed. To consider the intermediate principal stress effect on the rock mass strength, the unified elasto-plastic strength criterion is proposed in the nonlinear finite element analysis. The unified rock mass elasto-plastic material model is implemented in ABAQUS user interface. The reliability analysis is performed in two steps. Firstly, the failure response is obtained by fitting the limit state function of the rock mass using the quadratic polynomial based on the nonlinear finite element analysis of the rock mass. Secondly, the JC method is proposed to obtain the design point and the reliability index of rock mass. As a case for study, the reliability analysis of Jinhuashan railway tunnel within soft and weak rock mass is studied using the proposed method. The two dimensional analysis is performed to consider the excavation stage effect and the unloading effect on the rock mass. Computation result shows that the rock mass is in stability.