<p>Water–rock interactions triggered by rainfall and groundwater infiltration, as well as the geometric orientation of structural planes during geological history, are critical factors influencing the mechanical behaviour and long-term stability of rock masses with weak interlayers. Therefore, to investigate the individual and coupling effects of moisture content and dip angle, the laboratory experiments, digital image correlation (DIC) technology, and theoretical analysis are performed. Two distinct types of stress‒strain curves are identified, namely double-peak (Type I) and single-peak (Type II), and the coupling effect has a suppressing effect on the occurrence of the Type II curve at high moisture contents with high dip angles. Four robust statistical methods reveal the significance of the influences on triggering strength in the following order: moisture content &gt; dip angle &gt; coupling effect. Based on improved DIC technology, seven typical failure modes are categorized. The findings indicate that failures initiated by tensile or shear cracks (Mode I, III and V) occur first, followed by these modes (Mode II, IV and VI) dominated by strain gradient-driven cracks. The theoretical model of rock masses with inclined weak interlayer demonstrates that the weak interlayer acts as a core stress-regulating component. The stress concentration at the tips of the weak interlayer leads to tensile failure, whereas the region of maximum principal stress difference is the potential zone for shear failure. This research provides a critical theoretical basis for stability assessment, hazard prediction, and risk mitigation in engineering projects involving rock masses with weak interlayers.</p>