<p>Rockfill concrete (RFC) is constructed by initially placing large rocks and then filling the inter-rock voids with high-flow Self-Compacting Concrete (SCC) to ultimately form a complete concrete structure. The use of large-size rocks makes the structure of the interface transition zones and rocks interactions within RFC different from those of conventional concrete. The effect of the new internal structure on the overall mechanical performance of RFC is unclear. Moreover, the size of specimens for physical testing of RFC is much larger than that of conventional concrete, which hinders the test. This paper mainly utilizes numerical methods to carry out the related research. A three-dimensional mesoscopic finite element model (FEM) containing the rock, SCC, ITZ, and aggregate contact unit(ACU) between the rocks is established and validated combined with the experiments. After that, numerical modeling was used to investigate the effects of single changes in ITZ properties, aggregate properties and internal defects of aggregates on the uniaxial compressive properties of RFC. The results demonstrate that RFC’s uniaxial compressive performance increases with enhanced ITZ mechanical properties, although the role of ITZ in RFC differs from that in conventional concrete. RFC’s uniaxial compressive performance also improves with increasing aggregate mechanical properties, showing a strong linear correlation. The influence of aggregate properties on RFC is more significant than that of ITZ, highlighting RFC’s greater dependence on aggregate characteristics. Internal defects within the aggregate weaken its performance, thereby reducing RFC’s uniaxial compressive strength. As porosity increases, the degradation in RFC performance becomes more pronounced.</p>