Abstract: To investigate the characteristics of the AP/HTPB composition propellant at different heating rates in the slow cook-off, we constructed a a model consisting of two-dimensional unsteady-state heat transfer and two-step chemical reaction kinetics. The characteristics of the composition propellant at the heating rates of 3.3, 4.7 and 6 K/h were analyzed respectively. The results show that the ignition position occurs in the left side of the interface between the propellant and the gas. The heating rate has a great effect on the ignition time and the position for the composition propellant. As the heating rate increases, the ignition delay time decreases and the ignition position moves to the right. The change of the heating rate has limited impact on the ignition delay temperature.