Abstract: In reinforced concrete (RC) box structures, the blast wave is difficult to dissipate freely outwards, and the structure's damage degree can be intensified after multiple reflections. To thoroughly investigate the load characteristics and dynamic behavior of internal explosions in RC box structures, the internal explosion tests of fully enclosed and partially enclosed (with explosion venting) box structures were replicated. This process verified the applicability of the simulation material models and parameters, MAPPING method, and fluid-structure coupling algorithms. On this basis, for the prototypical RC box structures and the types of terrorist bombing attacks specified by the Federal Emergency Management Agency (FEMA), numerical simulations of internal explosions were conducted under three explosion threat scenarios and four venting areas. Furthermore, the load characteristic and its distribution at the structural inner surface centers and corners, as well as the structure's dynamic behavior, were analyzed. The results show that the venting area has a negligible effect on the overpressure, while the impulse decreases exponentially with the increase of venting area; the load distribution characteristics on the structure's inner surfaces are significantly influenced by the structural dimensions, exhibiting an 'indented' or 'W' pattern; the maximum displacement at the center of walls and slabs can be reduced by more than 50% for the venting coefficient increases from 0.457 to 1.220; the impulse criterion can more accurately assess the damage degree of components than overpressure. Finally, a calculation method for the impulse and damage enhancement coefficient considering the venting area was proposed, which could effectively predict the internal explosion load and structure's dynamic behavior at various venting coefficients.