Abstract: Thin spray-on liners (TSLs) are increasingly used in mining as a quick and effective method for sealing and stabilizing rock masses. Bond failure at the rock-TSL interface directly compromises the efficacy of the support system. To investigate the influence of interface roughness on the fracture characteristics of rock-TSL composites, red sandstone and mortar were selected as substrates, and cement-based TSL material was used to fabricate rock/mortar-TSL composite specimens. The fracture performance of these specimens under three-point bending was then examined. Combined with DIC technology, the evolution characteristics of strain and local displacement during the interface fracture process were analyzed. PFC^3D software was employed to analyze the crack propagation characteristics of composite specimens with different interface roughness. The results indicated that as interface roughness increases, the peak load, flexural strength, and fracture energy of the sandstone/mortar-TSL composite specimens all increase. When the interface roughness is the same, the bending resistance of the sandstone-TSL composite specimens is higher than that of the mortar-TSL composite specimens. When the roughness was 1.648 m⁻¹, the flexural strength of the rock/mortar-TSL composite specimens reached its maximum, with values of 5.997 MPa and 3.302 MPa, respectively. The failure of the sandstone-TSL composite specimens include adhesive failure at the interface between the TSL and the rock, as well as tensile fracture of the TSL. The failure of the mortar-TSL composite specimens is primarily interfacial fracture, with local tensile fracture of the TSL and surface spalling of the mortar. The strain map and displacement field obtained by DIC show that as the interface roughness increases, the maximum principal strain at the crack tip increases, and localized tensile displacement in the x-direction also increases. The PFC^3D simulation results show that with increasing interface roughness, the composite specimen exhibits a higher crack initiation load, an increased maximum contact force at peak load, and improved fatigue performance. For the same specimen, as the crack propagates, the maximum contact force at the interface increases before the peak and decreases after the peak. The results provide a scientific basis for evaluating the interfacial mechanical behavior of TSLs and the construction at over-excavated areas in mining engineering.