Abstract: At first, this paper discussed the calculation principle of stability coefficient of slope based on limit equilibrium method, and pointed out that the analytical solution can not be achieved due to the implicit expression of stability coefficient. Establishing the approximation standard for reliability degree, and according to the geometrical meaning of reliability degree, this paper developed an analytical approach to calculate reliability degree explicitly, in which, the limit state curve at a checking point is approximated by an explicit function, Detail implementation of the approach was presented through a calculation of slope failure probability considering the Spencer and Bishop model for limit equilibrium of slope. The calculation results were compared with the precise solution. The absolute error and relative error are 0.00379% and 8.42% for Spence model, and 0.0039% and 3.63% for Bishop model.