Influence of rainfall density,rainfall duration and perviousness of soil on changes of soil moisture are analyzed qualitatively.Relationship between the shear strength of loess and changes of soil moisture is also analyzed quantitatively.After the calculation on influence of different percolation depths on loess slope stability,a decision was gained that a certain slope carried with a certain critical percolation depth.One can estimate the stability of existing slope by compare critical percolation depth with the potential percolation depth that calculated by critical rainfall density and rainfall duration.One also can choose proper slope style by the calculating result.