It is advantageous to know how much rainwater decreases or evaporation increases the stability of a soil slope. Numerical model was used to quantify the effect of rainwater and evaporation on the stability of a case slope. With raining model and evaporation model

unsaturated-unsteady seepage analysis was used to calculate pore pressure of the case slope

and Bishop Equilibrium balance method with total cohesion was used to calculate the stability factor of the case slope. Relationships between the pore pressure and time

between the safety of stability and time were obtained. Results show that changes in slope stability and pore pressure lag behind the change of weather; infiltration and evaporation affect the stability of shallow slip surface greatly

the stability of deep slip surface relatively little; in the process of rain

the critical slip surface changes from deep surface to relative shallow surface

in the process of evaporation

the situation is opposite.