Burning or detonation reaction occurs after phase transition during cook-off for of low-melting point solid explosives. A thermal reaction model of explosive was established according to TNT cook-off test. Besides heat conduction, self-decomposing reaction, the phase transition and heat convection of liquid explosive were considered. The calculation was conducted to model the TNT cook-off by CFD software (Fluent). The ignition time (4150 s) and ignition temperature (226 ℃) are achieved. Comparing the calculated results with the measured, the calculation model and relevant parameters of TNT were validated. The phase transition and temperature distribution in explosive were respectively analyzed at 0.3 K·s^-1, 0.05 K·s^-1 and 3.3 K·h^-1 heating rate. Results show that phase transition occurs from the surface to the exterior of explosive. The unmelted explosive sinks in melted explosive because of the pull of gravity. The heat energy is needed for melting explosive. So the explosive temperature rises slowly. After all explosives have been melted, the temperature rises rapidly due to the convection. There is a little difference in temperature of liquid TNT because of heat conduction and convection. The ignition temperature, ignition time and ignition position of explosive are affected by phase transition of explosive.