Abstract: The stability of slopes along Qinghai-Tibet Railway and Highway is very important to their normal operations. The soil properties related to its analysis uasully have a high degree of uncertainty, and it is difficult to obtain sufficient statistical data to evaluate their variability. In this paper, based on Interval Theory, a new reliability analysis method of slope stability in permafrost regions with small sample size is proposed. The thaw slump of slopes is considered, and the performance function is given using the theory of infinite slope stability analysis. The uncertain variables are viewed as interval variables with variable bounds, and the upper bounds and lower bounds are the maximum value and the minimum value of sampling data, respectively. The ratio of the central value of the performance function to its dispersion is used as an index to assess the reliablity of slope stability. The index changes with the number of samples. In order to find out the relationship between the index and the probability reliability index, the uncertain variables are treated as random variables, and the formulation to calculate the interval of the ratio of the two indexes is given. The proposed method is used to estimate the reliability of a slope stability along Qinghai-Tibet Railway.