Traditional homogenization models have difficulty accurately capturing the mesoscale effects of aggregate distribution, aggregate particle size, and reinforcement configuration on stress-wave propagation paths and energy-dissipation mechanisms in concrete, thereby limiting an in-depth understanding of the blast-induced failure mechanisms of reinforced concrete slabs. To address this issue, a three-dimensional mesoscale finite element model of reinforced concrete slabs incorporating reinforcement, aggregates, and mortar matrix was established through the combined use of MATLAB and LS-DYNA. The aggregates were modeled according to actual aggregate gradation characteristics, the reinforcing bars were accurately arranged based on practical engineering layout parameters, and appropriate contact algorithms were adopted among the matrix, aggregates, and reinforcing bars to simulate interfacial effects. The model was validated against contact explosion tests, and the results show that it can predict the failure modes and crater dimensions of reinforced concrete slabs subjected to contact explosion loads with reasonable accuracy. On this basis, the effects of aggregate characteristics, including distribution pattern and particle size, and reinforcement arrangement on the blast resistance and stress-wave propagation behavior of reinforced concrete were investigated through parametric mesoscale numerical simulations. Regarding the aggregate parameters, the particle-size distribution pattern and aggregate size govern the evolution of stress waves and energy-dissipation characteristics, thereby affecting the geometric dimensions of the craters on the top surface and the spalling craters on the bottom surface of the concrete slab. When the aggregate particle size decreases from the top surface to the bottom surface, the expansion of the top-surface crater and the development of bottom-surface spalling can be effectively suppressed; in contrast, an increasing particle-size distribution aggravates surface cratering and internal spalling damage. In terms of aggregate size, the bottom-surface spalling craters of slabs containing small aggregates exhibit shallow and wide characteristics, whereas those containing large aggregates exhibit deep and narrow morphologies. Compared with aggregates, reinforcing bars exert a weaker influence on the overall failure mode and stress-wave propagation behavior of the slab. At a low reinforcement ratio, the reinforcing bars have little effect on the dynamic transmission of compressive stress peaks, whereas under high explosion loads, they suppress slab fragmentation, mitigate flexural damage, and improve the structural integrity and damage resistance of the slab.