<p>Evaluating interface treatments for ultra-high-performance concrete (UHPC)–normal strength concrete (NSC) composite beams remains a significant challenge in structural health monitoring of composite systems. Conventional monitoring methods are unable to effectively capture the full evolution of interfacial bonding behavior between UHPC and NSC. Acoustic emission (AE) based nondestructive testing provides a powerful approach for characterizing structural damage processes. In this study, four-point bending tests were performed to examine the interfacial bonding performance of UHPC–NSC composite beams with three surface conditions: no treatment, fine stone treatment, and epoxy resin treatment. The results demonstrate that the AE technique effectively characterized the debonding process in UHPC–NSC composite beams. Compared with untreated and epoxy treated specimens, fine stone treated beams exhibited more stable AE activity and a smoother, more controlled bottom-layer separation during debonding. AE ringing counts and energy were identified as reliable early indicators of incipient bottom-layer debonding. The fine stone treatment resulted in a smoother and higher-level b-value evolution with lower point density, indicating that damage was governed by gradual microcrack growth and coalescence, thereby leading to a more stable failure process. A higher proportion of the friction-related mid-frequency band suggests that fine stone treated beams possess superior interfacial bonding strength. Accordingly, the mid- to high-frequency ratio can serve as an effective indicator of interfacial bond quality. These findings provide a theoretical and technical basis for the practical engineering application of AE technology in monitoring the interfacial bonding performance of UHPC–NSC composite structures.</p>