The permeability characteristics of expansive soil have a significant impact on slope stability. This study investigates the permeability evolution of three expansive soils (B, GW, GB) with distinct swelling potentials (68%, 42%, and 28%) under repeated wet–dry cycles, combining laboratory falling-head tests and in situ four-ring infiltrometer measurements. The results demonstrate that the permeability coefficient increases consistently with cycle number. The high-expansivity soil (B) exhibited the most pronounced change, with permeability increasing from 10⁻⁶ cm/s to 10⁻³ cm/s after five cycles, whereas the low-expansivity soil (GB) remained stable. A significant specimen size effect was observed in initial permeability, which stabilized beyond a critical infiltration area of 1102.7 cm². However, this size dependence diminished after three cycles due to extensive cracking and structural homogenization. Crack density analysis further confirmed microstructural stabilization at sample diameters between 32 and 38 cm, validating the adopted in situ seepage diameter of 37 cm. Linear regression revealed greater discrepancies between laboratory and in situ permeability values in high-swelling soils. Empirical formulas incorporating wet–dry cycles were developed to accurately predict in situ permeability, providing practical tools for engineering design and risk assessment.