Abstract: In order to prevent the unstable damage of unsaturated red sandstone weathered soil slopes due to the changes in water state, a study of soil and water characteristics and stability changes under the action of rainfall was carried out. First, the dehumidifying test was conducted on the unsaturated red sandstone weathered soil under different initial dry densities and initial mass moisture contents, with soil and water characteristic curve obtained. By analyzing its rules, an improved exponential function model was proposed and compared with the F-X model. Finally, the model was applied to carry out numerical simulation, analyzing the effects of rainfall intensity and duration on slope stability. The results show that under the condition of fewer parameters and more concise expression, the improved exponential function model achieves a fitting accuracy close to that of the F-X model, and the estimation result of residual water content is more accurate, with better applicability to red sandstone weathered soil. When the rainfall holding time is the same, the rainfall intensity and slope saturation increases, but slope stability decreases. With the extension of precipitation, the slope stability is significantly reduced in 0‒6 h, and after that the stability is less reduced. The short period of heavy rainfall has a great influence on the stability of red sandstone weathered soil slope.