<p>Long-term corrosion and weakening by tailings water can reduce rock mass strength, potentially triggering engineering hazards such as tailings dam instability and slope failures. To investigate the effects of prolonged tailings water immersion on the mechanical properties and micro-damage mechanisms of sandstone, uniaxial compression tests, nuclear magnetic resonance (NMR), acoustic emission (AE) monitoring, and particle flow code two-dimensional (PFC^2D) discrete element simulations were conducted on sandstone samples subjected to different immersion durations (0–6 months). Results show that increasing immersion time raises porosity, shifts pore size distribution from micropores to meso- and macropores, and drives pore development from near-surface regions toward the rock core. In a weakly alkaline tailings-water environment, chemical weathering weakens the grain-boundary cementation of sandstone, causing crack modes to shift from splitting (tensile) to shear and triggering structural instability. This results in a progressive reduction of uniaxial compressive strength (UCS) and a reduction in AE activity, with the rate of decline diminishing over time. After six months of immersion, the elastic modulus and uniaxial compressive strength (UCS) of sandstone decreased by 36.28% and 35.60%, respectively, accompanied by increased plasticity. An AE-based damage model was established to quantitatively characterize the time-dependent deterioration. Immersion in tailings water strengthened the internal force chains, intensified strain localization, and increased both the total number of cracks and the proportion of shear cracks. These findings provide a scientific basis for the quantitative description of sandstone deterioration under tailings-water erosion and for the stability assessment of tailings dam engineering.</p>