<p>The macro-scale damage behavior of rocks is attributed to the accumulation of micro-scale damage. However, there is still a lack of comprehensive understanding of the tensile damage processes and modes of rocks at the micro-scale, especially for soft rocks. In this study, we used scanning electron microscopy and energy dispersive spectroscopy to analyze the composition and microstructure of slate. Clay mineral grains (MGs) are directionally arranged, forming bedding structures in slate, while non-clay MGs act as skeletons near the bedding. Fracture behavior and strength distribution of the interlayer MG interface in slate subjected to water–rock interaction are obtained using the single-MG interface mechanical test. Strength deterioration and microstructure models have been established to explain the quantitative relationship between microstructural evolution and strength. The results reveal that the crack path mainly propagates along the clay MG interface, controlled by MG size, shape, arrangement, and interface strength. Water–rock interaction leads to a reduction in the physical and the mechanical properties of slate. These factors lead specimens to exhibit catastrophic failure before water–rock interaction and non-catastrophic failure afterward. The tensile strength of MG interfaces exhibits an L-shaped decline, reaching a water softening critical state after 15&#xa0;days with a deterioration rate of about 51%, placing it within the early failure period. It is crucial that the clay content in MGs strongly correlates with the critical state of strength deterioration in slate. The findings from the meso-scale and micro-scale are insightful for understanding the macro-scale damage process of slate.</p>