Abstract: Weathering is a fundamental process governing the structural evolution and stability of rock masses in surface environments. Through coupled physical, chemical, and biological mechanisms, it alters the pore architecture of sandstone, thereby influencing its permeability and mechanical properties. To elucidate the pore-scale evolution of weathered sandstone and its impact on fluid flow, this study employed micro-computed tomography (μCT) to acquire high-resolution 3D images, reconstructed realistic pore structures using AVIZO, and conducted pore-scale flow simulations in COMSOL Multiphysics based on digital rock models. Results show that with increasing weathering intensity, sandstone exhibits a substantial rise in porosity, pore-throat radius, and connectivity, resulting in a more complex pore network and smoother flow channels. The simulated absolute permeability increases exponentially with weathering degree, showing good agreement with laboratory pressure-driven permeability tests. Pore network model analysis further indicates that highly weathered samples contain a larger proportion of high-velocity throats, confirming that enhanced connectivity and enlarged pore pathways are the primary mechanisms driving permeability evolution. This work quantitatively couples pore structure evolution and flow behavior of weathered sandstone across multiple scales, providing theoretical insights for predicting permeability variation in weathered rock masses.