Abstract: Clastic rocks are extensively distributed in the water-level fluctuation zone of the Three Gorges Reservoir area, with sandstone-mudstone interbeds being the most representative. Cyclic water-level fluctuations induce mudstone deterioration, leading to fracture initiation in the overlying sandstone layer under self-weight stress and consequently triggering localized collapses. This investigation examines sandstone specimens from reservoir bank slopes through systematically designed wetting-drying cycle tests. The experimental results demonstrate that: (1) All sandstone specimens with different crack lengths exhibit non-monotonic degradation in mechanical properties under wetting-drying cycles, characterized by initial enhancement followed by progressive deterioration in peak load, fracture toughness, and fracture energy, with longer cracks accelerating the degradation process; (2) Increasing wetting-drying cycles cause noticeable contraction of high-strain zones and significant crack-tip deflection, where specimens with 30-mm cracks display the most substantial reduction in high-strain concentration area and maintain the minimal crack-tip deviation; (3) An empirical fracture toughness model incorporating crack length and wetting-drying cycles was established based on continuum damage mechanics and energy release theory, showing excellent agreement between theoretical predictions and experimental measurements. This study elucidates the deterioration mechanisms of sandstone fracture properties under hydraulic cycles and provides a theoretical foundation for long-term stability assessment and hazard mitigation of sandstone-mudstone interbedded slopes in the Three Gorges Reservoir region.