The shallow instability of granite residual soil slope could easily induced by rainfall. Based on the analyses of influence of rainfall on the water content of slope soil

the effective rainfall resulting in soil from unsaturated to saturated condition is derived from the relationship between saturation and water content. Results show that not all rainfall will cause the instability of soil slope

and only when the soil saturation caused by rainfall infiltration reaches a certain degree the slope fails. There exists a critical depth in the unsaturated and saturated zone of slope above which the rainfall infiltration leads to a transition in soil from unsaturated to saturated

different shear strength could be used to calculate the stability of soil which was divided into fluctuation two parts by the critical depth. The soil unsaturated zone and saturated zone can be taken to calculate the stability of the soil condition. The results are useful for determining rainfall threshold of slope instability

so as to further understand the mechanism of landslide instability and landslide prediction.