Abstract: To address the insufficient interfacial bond performance in conventional carbon fiber-reinforced polymer (CFRP) strengthening systems and the incompatibility between traditional adhesives and the dense surface of ultra-high-performance concrete (UHPC), eighteen CFRP–UHPC interface specimens (including four control specimens) were tested under single-lap shear loading. The interfacial failure mode, load–slip response, CFRP strain distribution, and interfacial shear stress transfer were investigated, together with the effects of bond length, CFRP layers, and adhesive type. The enhancement mechanism of MMT/SiO₂ nano-modified epoxy was further clarified.The results indicate that interfacial debonding is the dominant failure mode, while the steel fiber bridging effect in UHPC suppresses crack propagation and results in progressive damage evolution. The load–slip curves exhibit evident post-peak softening behavior with considerable deformation and energy dissipation capacity. Increasing the bond length from 120 to 160 mm increases the ultimate load and ultimate slip by 37.7% and 41.1%, respectively, whereas further increasing the bond length to 200 mm reduces the corresponding increments to 23.3% and 17.0%, indicating a pronounced effective bond length. Increasing CFRP layers enhances the interfacial load capacity but aggravates stress concentration near the loading end, thereby accelerating interfacial softening. Compared with conventional epoxy, the MMT/SiO₂ nano-modified epoxy improves the interfacial cooperative deformation capacity, resulting in average increases of 40.9% and 31.7% in ultimate load and ultimate slip, respectively. The interfacial strain and shear stress exhibit nonlinear attenuation along the bond length, and the UHPC system presents a longer effective stress transfer zone and more uniform strain distribution. Finally, a fracture energy-based model for predicting the CFRP–UHPC interfacial capacity was established, and the predicted results agreed well with the experimental data, providing support for interfacial performance evaluation and composite strengthening design.