Aggregate and interfacial transition zone (ITZ) have significant influence on the carbonation resistance of cementitious materials. Based on Fick's second law, a CO₂ diffusion model considering the influence of aggregates and ITZ is proposed in this paper, the diffusion coefficient of interfacial transition zone is introduced for the first time and the calculation method of corresponding parameters in this model is given. At the same time, a CO₂ reaction model is established based on the law of mass conservation, and the CO₂ concentration distribution in C30 and C50 concrete under load is studied by using this model. The results show that the diffusion coefficient and depth of carbonation of CO₂ in cementitious materials can be calculated more accurately by considering aggregate and ITZ effects. Finally, the reliability of the theoretical model is further verified by comparing with the experimental data in literature. Therefore, the model takes aggregate and ITZ as important factors to evaluate the durability of concrete, which provides foundation for the design and life prediction of reinforced concrete structures.