Abstract: The stability of completely weathered granite is poor and it is easy to disintegrate when exposed to water. In engineering, the grouting effect of using conventional materials for seepage prevention and reinforcement in this rock is poor. In view of this situation, based on the grouting project of the Mangshan reservoir in Chenzhou of Hunan Province, the simulated grouting experiment is carried out by the independent design of the laboratory simulation test device. The diffusion of slurry in the whole grouting process is simulated. Uniaxial compression, shear strength and permeability tests are conducted on the samples taken at different grouting pressures and at different locations, and the whole stone body under different grouting pressures is sampled and observed to examine the anti-seepage reinforcement effect and the slurry diffusion mode of the high-solid phase segregation slurry, which is composed of fully weathered granite particles, in the fully weathered granite. The results show that the slurry has undergone three stages of permeation and diffusion, compaction and compression, and cleavage and expansion in the process of fully weathered granite formation diffusion, and the high solid phase segregation grout with fully weathered granites as the main body has a remarkable effect on the grouting of fully weathered granites. With the increasing grouting pressure, the uniaxial compressive strength is significantly increased to 3.25-13.67 times the original soil mass, and the shear strength is increased to 1.63-2.69 times the original soil mass under different normal pressures. The permeability decreases from 10⁻⁴ cm/s to 10⁻⁵ cm/s or 10^−-6 cm/s. Therefore, the high solid phase segregation slurry has a significant anti-seepage effect on the fully weathered granite.