Abstract: To elucidate the damage mechanisms of the interfacial shear performance of historic masonry under hygrothermal conditions, a multi-cycle hygrothermal accelerated aging test was designed based on the climatic characteristics of Shaanxi Province, and double-shear tests were conducted on historic masonry specimens. Based on the experimental results, the evolution of shear failure modes and interfacial shear mechanical properties under different hygrothermal cycles and pre-compression stress levels was comparatively analyzed. Furthermore, a constitutive model for interfacial shear damage of historic masonry under coupled hygrothermal effects was established based on statistical damage theory. The results indicate that pre-compression stress significantly enhances the interfacial shear performance by improving interfacial friction and mechanical interlocking and serves as the dominant factor affecting interfacial shear strength, with a contribution ratio of 87%. In contrast, the contribution ratio of hygrothermal cycling is 8%, although it exhibits a stage-dependent influence on interfacial performance. During the initial stage, mortar carbonation increases the peak shear strength by approximately 17%. With continued cycling, the peak shear strength gradually decreases and the peak slip increases, indicating evident cumulative damage. The interfacial shear damage constitutive model established based on statistical damage theory agrees well with the experimental results and can accurately characterize the full-stage shear stress-slip response of historic masonry interfaces. The findings provide a mechanical explanation for the evolution of interfacial shear damage in historic masonry under hygrothermal conditions.