Taking the moisture content as main control parameter of shear strength of the soil

we analyze the variational properties of moisture content and shear strength with the change of rainfall time during the small sustained rainfall infiltration

and establish the relationships between total cohesion as well as effective angle of friction and the time of sustained small rainfall infiltration into an unsaturated slope. With the rainfall infiltration

the influence area of slope expands. Through the strength reduction method based on FLAC3D

the dynamic safety factor of slope in different time and the duration time before the failure of the slope are obtained. The results show that the slope critical sliding surface is yet unsaturated during a sustained small rainfall.