Abstract: The stability of soil nail structure was determined by the interaction between soil nail and soil mass.On the one hand,the characteristic of the interaction was influenced by many factors(maybe called as random variables in the mathematics),such as the properties of soil mass,length and inclination of soil nail,the strengthened depth and so on;on the other hand,the compound mode of those influencing factors in the design and the difference of theoretical model should not be ignored,So,a state equation of limit equilibrium,which was applied to stability analysis of soil nail structure,was set up considering the influence of inter-slice force and combining the idea of disturbance force of Sarma with slice method.In the design of timbering of excavation,the reliability theory was used to analyze not only the sensitivity of random variables,including index of reliability and safety factor,but also the relationship between them.The theoretical model and its application were shown in a practical project.