<p>Expansive soils, as a special soil type with significant expansion and contraction characteristics, desiccation cracking in expansive soil critically influences expansive clay slope instability and infrastructure foundation stability. Desiccation cracks alter the pore structure of expansive soils and significantly affect their infiltration characteristics. Nevertheless, understanding the three-dimensional evolution patterns of internal cracks and their influence on soil infiltration characteristics remains limited. This study employed high-precision X-ray computed tomography (X-CT) and 3D numerical simulation to track 3D desiccation crack evolution and soil permeability dynamics, and infiltration tests were performed to validate the seepage simulation results. Results demonstrated that with prolonged drying, soil shrinkage intensified and crack density progressively increased, the total crack volume expanded from 628.35 mm³ to 1731.8 mm³ with a growth rate of 175.6%, crack ratio and connectivity in expansive soils increased significantly, while their coefficient of variation grew. According to their connectivity, cracks were categorized as interconnected or isolated, the interconnected cracks volume increased by 190.7%, and the isolated cracks volume increased by 56.9% as moisture content decreased from 25.0% to 10.0%. Initiation, propagation, and coalescence of desiccation cracks‌ substantially enhance flow path density and velocity, markedly increased the permeability coefficient. Combined simulations and experiments verified that interconnected cracks primarily govern expansive soil permeability characteristics. This study advances theoretical foundations and provides practical insights into relationships between 3D crack-structure characteristics and permeability in expansive soils.</p>