A thermal reaction model of explosive was established for the cook-off test under the thermal condition of fire and thermal stability consideration of the explosive with the unexpected thermal radiation. The RDX-based cast cured alumnized （GHL） explosive was selected to investigate the characteristics of thermal ignition of the explosive at heat flux of 10⁴,10⁵,10⁶ W·m^-2, respectively. The calculation is conducted by means of the CFD software (ANSYS FLUENT) and the time to ignition, the ignition temperature and the ignition location were obtained. Results show that the ignition time of explosive decreases and the shell temperature increases with increasing of heat flux. The ignition temperature is slightly affected by heat flux. The ignition location is center of the upper center of the explosive.