Abstract: Due to the accidental explosion of a large-scale aboveground ammunition storage magazine, the dynamic response under internal blast loading, especially the size and launch velocity of debris of reinforced concrete structures, can cause huge damage to surrounding persons and buildings. Therefore, predicting accurately the debris formation and launch velocity of RC structures has become an important issue for the safety assessment of ammunition storage magazine. In this paper, a numerical model of RC slab bolted to the cubic steel box at the four ends is established with the explosive charge placed at the centre of the chamber. The numerical model adopts a three-curve concrete strength model to account for the effect of strain rate on the concrete and reinforcement dynamic strength. The air-structure coupled interaction is also considered. With the numerical model, the whole evolving process is simulated including break-up, debris formation and launching of RC slab. The simulation results are compared with experimental results, and good agreements are observed in the launch velocity and sizes of debris. It is also found that the shock wave load is responsible for the development of damage zone in the concrete material and the subsequent debris size distribution, and the gas pressure load plays an important role in the separation of crack pattern and in the subsequent throw of the debris.