Abstract: This paper analyses and studies the three dimensional sliding mass spatial shape by examining the 62 landslide data on the no-working slope of Hanzhou Surface Coal Mine. The properties of rock massare looked upon as the random field with spatial variability. The formulae calculating critical slidingmass length and three-demensional reliability indices are obtained by means of reliability theory. Theseformuale satisfactorily exlain why the ratio of the length to the height of the sliding masses those landslides occur on soft and weak layers is larger. They may be used to quantitatively calculate sliding mass lengths and three-dimensional reliability indices.