The cement paste-aggregate interfacial transition zone (ITZ) is a key component in concrete, and its formation is related to the sidewall effect. The formation of ITZ can also be affected by raw material properties and ratios. A numerical model of ITZ is developed based on the corpuscular principle, the continuous slicing method, and the cement hydration theory. The effects of the non-uniform distribution of cement particles and local differences in water-to-cement ratios(w/c) due to wall effects are comprehensively taken into account. The present model can be used to research the microscopic characteristics of ITZ, such as the thickness and porosity, and its reliability is verified by third-party experimental data. Based on the proposed model, the w/c, cement particle size distribution, and aggregate surface roughness were analyzed parametrically. The results show that, though the hydration degree of the cement in ITZ is high, its hydration product content is significantly lower than that of the cement paste matrix. The larger the w/c, the higher the thickness and porosity of ITZ. Finer cement particles can fill the gaps between the large particles, thus reducing the ITZ thickness and porosity. Rough aggregate surfaces, on the other hand, can enhance the wall effect and increase the thickness and porosity of ITZ.