A cook-off model of explosive was established to investigate the characteristics of explosive thermal reaction at different heating rates. The numerical simulation was employed to model the cook-off process for GHL explosive at different heating rates by computational fluid dynamics(CFD) software. The decomposition reaction of explosive was described by Arrhenius-equation. According to T-t curves of cook-off test at heating rate of 1 K·min^-1,the activation energy and frequency factor were confirmed. The calculations were conducted to simulate the cook-off tests for GHL explosive at heating rates of 3.3 K·h^-1,1 K·min^-1,3 K·min^-1 and 10 K·min^-1,respectively. The results show that the external heating rate has much influence on the ignition time and ignition location. The ignition time of explosive decreases and ignition location removes from the interior to the edge of explosive cylinder with increasing of the heating rate. The ignition temperature is slightly affected by the heating rate. The environment temperature at igniting in slow cook-off test is lower than that in fast cook-off test.