The shallow landslides are often triggered by heavy rainfall in weathered granite soil slopes. These slope failures usually occur in a strongly weathered granite layer which has a big pore size. These can cause great casualties and property losses. Through field investigation and laboratory experiments, the effect of saturation on the shear strength of highly weathered soils and completely weathered granite soils taking from the intersection of Yulin and Wuzhou are investigated. It is found that there exists an "optimal saturation" at which the shear strength of the soil reaches the maximum. The influence of saturation on both cohesion and internal friction angle does not show a unique trend. It is also shown that the cohesion is significantly effect by the saturation of the soils; but the friction angle is not. Completely weathered granite soils and the highly weathered granite soils are significantly different in their lithological, composition, construction and shear properties. The characters of the highly weathered soil and completely weathered granite soil are significantly different, especially in the relationship between saturation and shear strength .Based on the analysis of cementation effect and the principles of matric suction, a sound explanation for the mechanisms underlying the above-observed discrepancy is provided.