Abstract: Weathering modifies the microscopic pore architecture of sandstone through coupled physical, chemical, and biological processes, thereby exerting a primary control on permeability and associated mechanical behavior. In this study, high-resolution three-dimensional images are acquired by micron-scale X-ray computed tomography (μCT), and the pore structures of sandstones with different weathering degrees are reconstructed using AVIZO. Digital core models are then implemented in COMSOL Multiphysics to perform seepage simulations. The results indicate that increasing weathering leads to systematic increases in porosity, pore-throat radius, and connectivity, accompanied by a more complex pore network and better-developed flow pathways. Consequently, the simulated absolute permeability increases markedly and is consistent in order of magnitude with laboratory confining-pressure seepage test results. Pore-network analysis further shows that highly weathered sandstones exhibit a higher proportion of high-velocity throats, demonstrating that pore enlargement and enhanced connectivity are the dominant mechanisms responsible for the permeability enhancement during weathering. These findings provide a quantitative basis for interpreting seepage mechanisms in weathered sandstones and for predicting their hydraulic performance in engineering applications.