Abstract: Given the prevalent issues of weak interfacial bonding in urban stone slab pavements and insufficient shear resistance of conventional grouting materials—both of which contribute to premature pavement deterioration—there is an urgent need to develop advanced grouting materials with enhanced bonding strength and superior shear resistance, as well as to elucidate their underlying interfacial reinforcement mechanisms. This study focuses on polymer-modified ultra-high-performance concrete (UHPC) and systematically conducts shear strength tests to comparatively evaluate the effects of interface roughness (smooth vs. rough) and grouting material type (ordinary cement mortar, polymer-modified POM-UHPC, and polymer-modified UHPC) on interfacial bonding performance. The results indicate that polymer-modified UHPC specimens achieve peak shear strength at 28 days, exhibiting increases of 89.2% and 71.7% over ordinary cement mortar and polymer-modified POM-UHPC, respectively. When used as a jointing material, polymer-modified UHPC significantly enhances the interfacial bond between the substrate and stone slabs. Relative to smooth interfaces, roughened interfaces increase the 7-day shear strength of ordinary mortar, polymer-modified UHPC, and polymer-modified POM-UHPC by 16.0%, 21.5%, and 19.8%, respectively, thereby effectively accelerating early-strength development. Among the tested materials, polymer-modified UHPC demonstrates the most pronounced improvement, indicating its superior early-stage interfacial enhancement capability. Enhancing stone slab surface roughness substantially activates the synergistic strengthening effect of polymer-modified UHPC, resulting in simultaneous improvements in both interfacial bonding performance and shear strength, thus extending pavement service life. These findings confirm the strong potential of polymer-modified UHPC as a high-performance joint filling material for stone slab pavements.