Abstract: In response to the problem that bentonite, widely used in industrial pollutant containment, often suffers from significantly reduced swelling and permeability performance due to erosion by alkaline wastewater, which affects the long-term service performance of anti-seepage barriers, NaOH solutions were utilized to simulate alkaline conditions. Sodium bentonite was employed as the research object, and tests including swell-consolidation and unconsolidated-undrained direct shear tests were conducted. The influence of alkali solution concentration on key mechanical properties of the specimens, such as swellability, shear strength, and impermeability, was investigated. The mechanism behind the performance evolution was analyzed by combining micro-testing techniques including X ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP).The results indicate that with increasing NaOH concentration, the maximum swelling rate of the samples gradually decreases, shear stress and shear strength enhance, while the permeability coefficient increases significantly. XRD analysis shows that the montmorillonite content decreases with rising alkali concentration, leading to reduced swelling capacity. Meanwhile, an increase in osmotic suction results in higher effective stress, further suppressing swelling. SEM images reveal that the alkaline solution enhances cementation between montmorillonite particles, transforming the flaky structure into blocky and plate-like stacks with enlarged inter-aggregate pores, forming more permeable flow channels. MIP results further confirm the transformation of pore structure from small to larger pores, thereby enhancing permeability. Through a combined macro- and micro-scale experimental approach, this study elucidates the internal mechanisms of montmorillonite mineral transformation and pore structure evolution under alkaline conditions, providing important theoretical support and parametric references for the safe design and performance prediction of sodium bentonite in hydraulic barriers for alkaline industrial contaminated sites and nuclear waste repositories.