The influence of temperature rise on the safety of main charge of naval mine is studies in this paper. RDX-based PBX explosive was selected as the main charge, and the simulation calculation was carried out by Fluent software. The influence of increased temperature and charge size on the thermal explosion of explosive during the cook-off were emphatically analyzed, which provided an effective basis for improving the safety of main charge of naval mine. The results show that the explosion delay time of the charge decreases exponentially with the increase of temperature during constant temperature of the cook-off. The ignition temperature of the charge is not related to the heating rate. With the increase of the heating rate, the explosion delay time of the charge decreases nonlinearly. At different heating rates, with the increase of charge size, the ignition position moves gradually from the center to the edge of charge. In addition, with the increase of heating rate, the ignition time decreases gradually, and the charge diameter has little effect on the ignition time. The ignition temperature decreases first and then increases. The relationship between charge diameter and ignition temperature is non-linear.