To investigate the thermal safety of modular charge, based on the chemical reaction mechanism of combustible cartridge material and single-base propellant, a two-dimensional unsteady state cook-off model of modular charge was established. The fast cook-off response characteristics of modular charge were analyzed at the external heating rates of 1-10 K·min^-1. The results show that the initial ignition position of modular charges is all in single-base propellant on the left and right sides near the inner wall surface of combustible cartridge material, and the ignition zones are two circular response zones. With increasing the heating rate, the circular response zone will move from the inside of single-base propellant to the inner wall of combustible cartridge material, but the change in external heating rates has little effect on the ignition position of modular charge. At the heating rates of 1, 6 K·min^-1 and 10 K·min^-1, the temperatures occurring the cook-off response of single-base propellant are 458.2, 453.9 K and 455.7 K, respectively, which are basically consistent with the ignition temperatures (443-463 K) measured by experiments. The change in external heating rate has little effect on the ignition temperature occurring the cook-off response of modular charge, but the ignition time occurring the cook-off response of modular charge reveals an exponential decay with increasing the heating rate.