<p>Foamed polymer materials, known for their high expansion ratio, excellent impermeability, and environmental friendliness, are extensively applied in grouting repair engineering. However, the diffusion behavior within fracture networks is complicated by the expansive nature of polymer slurry. To address this, fracture curves with varying roughness were established, and a numerical model for polymer slurry diffusion in fracture networks was developed. The sealing rate and diffusion pressure distribution at fracture intersections were analyzed by adjusting the intersection angles within the network. Additionally, a physical model apparatus was designed to simulate flowing-water grouting under different fracture roughness conditions. Using an orthogonal experimental system, the dynamic diffusion and sealing mechanisms of polymer slurry in rough fracture networks were systematically investigated. Finally, scanning electron microscopy was employed to observe the microstructure of the solidified slurry under different conditions. The study reveals that: (1) The diffusion behavior observed in the polymer fracture network grouting model aligns well with experimental results. As the intersection angle of fractures increases, the diffusion pressure at the intersection rises, with the best sealing effect observed at a 60° intersection angle. (2) Increased fracture roughness significantly reduces the slurry sealing efficiency. (3) The sealing efficiency is correlated with the sealing area ratio, though variations arise depending on the slurry’s diffusion path and its effectiveness in sealing water flow channels. (4) SEM analysis reveals that the solidified structure near the grouting hole is denser with smaller pores, while farther away, the pores enlarge, and the surface becomes rougher.</p>