Abstract: To investigate the diffusion mechanism throughout the entire process of rock-fracture repair by polymer slurry grouting, a theoretical model for polymer slurry grouting diffusion in a pipe–parallel-plate single fracture was derived based on the expansion characteristics of the slurry and viscous fluid mechanics theory, with comprehensive consideration of the diffusion process and changes in fluid properties during the static-pressure diffusion stage and the self-expansion stage. Meanwhile, visualization grouting tests using parallel-plate fractures were conducted to validate the theoretical model, and the differences between the calculated results of the two theoretical models with and without consideration of the static-pressure diffusion stage were comparatively analyzed. The results show that the calculated slurry diffusion distances and pressure losses under different test conditions agree well with the experimental results, with all errors within 20%, demonstrating the validity and accuracy of the established theoretical model. The grout-front diffusion distances and pressure losses calculated by the theoretical model that does not consider the static-pressure diffusion stage deviate considerably from the experimental values, and the calculated results deviate further from the actual values as the injected grout mass increases.