Corrosion of steel bar induced by chloride salt attack is one of the main factors leading to the failure of recycled concrete. Hence, it is of great significance to study the microscopic diffusion behavior of chloride ions in recycled concrete for evaluating the durability of recycled concrete in marine environment. A new mortar layer is applied on the old mortar layer whose thickness changes, and the thickness changes of the old mortar layer are assumed to conform to the normal distribution of random number, so as to construct transition zones of different states between the new and old mortar interfaces. On this basis, the meso-structure of five-phase regenerated aggregate, namely coarse aggregate, new-old mortars and the transition zones of new-old interface, was established. Based on the diffusion theory, the chloride diffusion coefficients of each component was obtained, and the mesoscopic numerical model of chloride ion diffusion in the recycled concrete was established. The reliability of the chloride ion diffusion model of recycled concrete is verified by comparing with the existing experimental data. The effect of recycled coarse aggregate, old and new interface transition zone thickness, relative diffusion coefficient of the old interface transition zone, relative diffusion coefficient of old mortar and the adhesion rate of the old mortar on the chloride ion diffusion behavior were further studied. The results show that: compared with the natural aggregate, the adhesion of old mortar weakens the blocking effect of the regenerated coarse aggregate on the chloride ion attack. Although the area proportion of the interfacial transition zone is very small, however, the resistance to chloride ion attack of the reclaimed concrete decreases with the increase of the interfacial transition zone thickness and the diffusion coefficient. In addition, the chloride ion diffusion performance of the recycled aggregate concrete is enhanced with the increase of the adhesion rate and diffusion coefficient of the old mortar, and the enhancing trend is more significant with the increasing volume fraction of the reclaimed aggregate. Therefore, in order to prolong the durability of recycled concrete, it is of great significance to improve the anti-chloride ion attack of the interfacial transition zone and the old mortar, as well as to reduce the adhesion rate of the old mortar.