<p>Interface roughness is the key factor to control the mechanical behavior of rock concrete composite, but few studies have revealed its meso mechanism under compressive load. In this study, four kinds of natural rough interfaces (<i>JRC</i> = 10.72–16.52) were accurately reproduced by using 3D scanning and digital engraving technology. The synergistic effect of roughness and confining pressure was quantified by systematic triaxial compression test combined with multi-scale monitoring method. The core findings show that: (1) there is a critical <i>JRC</i> threshold (about 15). When the confining pressure is 20&#xa0;MPa, the peak stress increases by 83%, and the Hoek - Brown parameter <i>m</i>_<i>b</i> surges by 134%. Beyond this threshold, the marginal effect appears due to stress localization. (2) Compared with the smooth interface, the peak principal strain of <i>JRC</i> = 16.52 interface increased by 285%. (3) Acoustic emission characteristics showed that the failure mode changed from tensile dominant to shear dominant, which showed that RA value increased and AF value decreased. (4) DIC analysis reveals the “geometric anchoring” mechanism, that is, roughness controls crack path selection through strain localization and interface stress inversion. In this study, a quantitative framework of “interface roughness - confining - pressure” performance is established, and the research results provide a theoretical basis for the quantitative design and optimization of interface stability in related projects.</p>