:The change of saturation is only the external macroscopic expression of the change of shear strength of completely weathered granite soil, while the change of matrix suction is the key factor of the change of shear strength of rock and soil mass. In order to study the influence of matrix suction on the shear strength of fully weathered granite and provide data reference for actual slope and dam projects, the centrifugal method was used to conduct experimental research, and the VG model was used to fit the experimental data to study the matrix suction of fully weathered granite with different levels of composition and different water content. The soil and water characteristic curves (SWCC) of weathered granite with different levels of composition were obtained. The shear strength of weathered granite with different saturation was determined by direct shear test. The results show that the saturation of the completely weathered granite soil is negatively correlated with the matric suction. Soil samples with less uniform gradation have better drainage capacity. The influence of matric suction on the shear strength of completely weathered granite soils is different, and the cohesion is more sensitive to the change of matric suction than the internal friction Angle. The scanning electron microscope analysis shows that the porosity of soil samples is one of the factors affecting the matrix suction. When the saturation of the weathered granite soil samples reaches a certain degree, cement will be generated to fill the pores, which increases the matrix suction required for water discharge. There is a positive correlation between the matrix suction and the shear strength in the weathered granite soil. The greater the matrix suction is, the stronger the shear strength is.