Abstract: Fast cook-off assessment has always been a crucial topic in ammunition safety research. However, previous simulation studies typically did not consider the influence of factors such as outer casing ablation on ammunition ignition behavior. This paper pre-sets different casing ablation hole sizes and gap widths, employing the computational fluid dynamics methodology to obtain the flame temperature field under ablation conditions. The flame thermal load is then transferred as a boundary condition for finite element method calculations, thereby investigating the ignition behavior of the explosive after casing ablation. Simulation results indicate that when the casing gap is 5 mm, the ignition position of the explosive varies with the size of the ablation hole, consistently located at the edge of the hole. When the casing gap reaches 10 mm or more, the ignition position does not change with the ablation hole size and is consistently located directly above the explosive. On the other hand, the size of the casing ablation hole and the casing gap have no significant effect on the ignition time of the explosive. Mechanism analysis suggests that the ignition position is influenced by the reaction between the residual fuel in the high-temperature gas and the air within the casing gap. This study can provide a theoretical reference for accident emergency response, ammunition safety assessment, and improvement.