Abstract: During the grouting process in completely weathered granite strata, the water phase in the grout under? goes a softening effect under the constraints of grouting pressure and geotechnical boundaries, which affects the grout diffusion in the three stages of penetration, compaction, and fracturing to varying degrees. This study first clarifies the softening effect in the grouting diffusion process in completely weathered granite strata and explores its impact mechanism. A theoretical model for three-stage grout diffusion in completely weathered granite strata considering the softening effect is established. The feasibility of this theoretical model is then validated through physical model experiments. Finally, the research results are successfully applied in the grouting project of Line 18 of the Guangzhou Metro. The results show that the softening effect accompanies the entire grouting process in completely weathered granite strata, with the error between theoretical derivation and physical model test results being less than 20%,proving the feasibility of the derived theoretical model. The on-site grouting results indicate that the unconfined compressive strength of the rock core soil is greater than 0.8 MPa. After tunnel excavation, the working face remains stable, forming a main grout vein and branching grout veins with good ductility, and no disaster accidents occurred during subsequent excavations.