<p>To investigate the effects of wet–dry (W–D) damage and interlayer size on the shear strength and fracture morphology of rock materials. Using nuclear magnetic resonance (NMR) technology to test the pore structure and water absorption distribution of sandstone, interlayer materials, and interlayer sandstone samples under W–D cycles. Subsequently, acoustic emission (AE) technology and digital image correlation (DIC) technology were combined with a shear system to conduct shear tests, testing the shear strength of rock materials after W–D damage. Finally, using optical 3D scanning technology, a digital topographic map model of the shear fracture surface was established, and the effects of W–D cycles and interlayer size on the geometric shape of the shear fracture surface were studied. By calculating the fracture roughness indices of three types of rocks, the influence of W–D cycles and shear strength on the roughness was quantitatively studied. The research results show that the shear strength and roughness indicators of the rocks decrease with increasing number of W–D cycles. The analysis of the four roughness indicators indicates that the roughness indices of the three types of rocks decrease linearly with the decrease in shear strength. This means that it is feasible to estimate the shear strength of rocks using the roughness index of the rock fracture surface. This study provides a theoretical basis for further exploring the relationship between rock fracture morphology indicators and rock mechanical parameters.</p>