Abstract: When subjected to bridge-deck contact explosions, reinforced concrete (RC) box girders with closed cross-sections are exposed not only to external blast loads but also to internal loads generated by shock waves entering the cell after deck perforation. To investigate the propagation mechanism of these inflowing shock waves and the resulting distribution of internal blast loads, the adopted finite element method was first validated against existing RC slab contact-explosion tests and confined internal-explosion tests by comparing both local damage characteristics and shock-wave propagation behavior. The validated method was then used to investigate shock-wave evolution and bottom-slab blast load distributions in a prototype box girder under four representative threat levels, as well as the influence of end boundary conditions. The results show that shock-wave propagation comprises four stages: leakage into the cell, regular reflection, irregular reflection, and planar-wave propagation. The blast loads on the inner surfaces of the box girder generally decrease with increasing propagation distance. Transversely, wave convergence significantly amplifies the loads near the cell corners. Longitudinally, multiple reflections and wavefront reshaping transform the shock wave into an approximately planar wave, producing a propagation pattern similar to that observed in tunnel explosions. Compared with the open-end condition, the closed-end condition causes full cross-sectional reflection of the planar wave at the end plate, thereby increasing the load intensity in the adjacent region. Finally, empirical predictive models for the peak overpressure and positive impulse on the bottom slab were developed through regression analysis, enabling rapid estimation of internal blast loads on RC box girders subjected to bridge-deck contact explosions.