The bonding performance between UHPC (ultra-high-performance concrete) and existing concrete substrates is a critical factor in structural reinforcement and repair. This study experimentally investigates the effects of failure mode, matrix strength, interfacial roughness, and curing age on splitting tensile strength and double-sided shear strength, while analyzing the micro-mechanisms of the interfacial roughness group. Results indicate that: failure primarily occurs through interfacial delamination; increased matrix strength and curing age significantly enhance interfacial bond strength; interfacial bond strength continuously increases with rising interfacial roughness but stabilizes beyond RO2; top-pouring methods yield higher bond strength than side-pouring; the RO2 group achieves optimal interfacial bonding performance by balancing mechanical interlocking and chemical bonding through moderate roughness, forming a dense interfacial transition zone.