To address the significant deviation in using wet-sieved two-graded concrete to represent the chloride ion diffusion coefficient of three-graded concrete in ship lock engineering, chloride ion diffusion models for both wet-sieved two-graded concrete and three-graded concrete were established. On this basis, the effects of interfacial transition zone (ITZ) thickness and coarse aggregate volume fraction on the chloride ion transport behavior in three-graded concrete were explored. Additionally, a comparative analysis was conducted on the differences in chloride ion diffusion behavior and durability life prediction between wet-sieved two-graded concrete and three-graded concrete. Simulation results indicate that the chloride ion diffusion coefficient of three-graded concrete decreases significantly with increasing coarse aggregate volume fraction and ITZ thickness. Among the aggregate dilution effect, aggregate tortuosity effect, and ITZ effect, the ITZ effect plays a dominant role. Furthermore, the chloride ion diffusion coefficient of wet-sieved two-graded concrete during 720, 1 800 d of corrosion is 1.77, 1.86 times that of three-graded concrete. And using wet-sieved two-graded concrete to represent three-graded concrete for life prediction may lead to underestimated evaluation results.