The geological hazard of soil-rock mixture slope is one of the most common geological hazards in China and rainfall is the main inducing factor. The study of slope degradation mechanism under rainfall conditions is an important research direction in the field of disaster prevention and mitigation. By analyzing the experimental data of the mechanical properties of soil-rock mixtures under different water content conditions, the evolution equation of shear strength parameters with moisture content is established. A study is implemented that considering the influence of the weakening of mechanical parameters under rainfall conditions for the typical soil-rock mixture slope in Southwest China. The analysis of slope stability is carried out using Geostudio software. The results show that the evolution equation can effectively reflect the weakening effect of the shear strength of the soil-rock mixture caused by the increase of the water content.the increase of the rainfall intensity will significantly reduce the slope stability while the rainfall duration is only a reflection of the value at a certain time in the rainfall process which has a little effect on slope stability. The rainfall and slope safety factor are obviously linear and negatively correlated. The research results can provide a technical support for the prediction and prevention of geological disasters on soil-rock mixed slopes.