Based on the Monte Carlo method, Python was used for secondary development through the Abaqus interface to generate a two-dimensional microstructural five-phase model for Recycled Aggregate Concrete (RAC), distinguishing between natural coarse aggregates, the transition zone (ITZ) between new and old interfaces, and new and old mortar. An improved moisture-chloride ion coupling model under dry-wet cycles was proposed, and the computational results of this model were compared and validated against physical experiments, with good agreement. This model was then applied to analyze the effects of dry-wet cycle periods, ITZ permeability, water-cement ratio, and natural aggregate volume fraction on chloride ion transport properties.The numerical results showed that as the number of dry-wet cycles increased, the diffusion depth and concentration of chloride ions in RAC also increased. When the ratio of ITZ diffusion coefficient to the new mortar diffusion coefficient increased, the chloride ion concentration in the diffusion region increased significantly, especially at the front end of the diffusion zone (at a distance of 12.5 mm from the starting point). In addition, there was a positive correlation between RAC materials with different water-cement ratios and chloride ion transport capacity, with little variation in chloride ion transport performance within the high water-cement ratio range. Finally, the volume fraction of recycled aggregates had a significant impact on the chloride ion permeability of RAC, indicating that the ITZ and new and old mortar have an important influence on the transport of chloride ions