Based on the Monte-Carlo principle and Fuller grading theory, a three-dimensional spherical mesoscale model of recycled concrete, composed of natural aggregates, old interface zones, old mortar, new interface zones, and new mortar, was established using PYTHON algorithms and COMSOL software. This study investigates chloride ion transport mechanisms within recycled concrete through modeling and analysis, with particular focus on elucidating the influence of material composition on erosion resistance. The three-dimensional spherical aggregate distribution model developed using stochastic theory effectively captures the authentic mesostructural characteristics of recycled concrete, with validation results demonstrating the model’s reliable engineering applicability. The chloride ion mesoscale numerical simulation results based on this model were consistent with experimental results. When the recycled aggregate volume fraction increased from 10% to 30% and 50%, the apparent diffusion coefficient of recycled concrete increased by 6.56% and 19.43%, respectively. When the old mortar diffusion coefficient increased fourfold, the apparent chloride ion diffusion coefficient increased by 19.28%. When the old mortar thickness doubled, the apparent chloride ion diffusion coefficient increased by 8.1%. Increasing the thickness and diffusion coefficient of the old mortar weakens the recycled concrete’s resistance to chloride ion erosion.