Abstract: The influence of underground water on the stability of joint slopes was studied completely by the synthetic method of qualitative and quantitative analyses. First, a numerical simulation of underground water flowing in a slope was made by the dispersed element method. Then, an uncertain method-fuzzy point estimation was used to analyse the reliability of the slope under the conditions of water ignored and considered, respectively. The results showed that the failure area extended, the failure probability increased greatly, and the reliability index decreased greatly when water was considered. It is concluded that water has unignored effect on slope stability.