Aiming at the combustion destabilization during the depressurization of base bleed projectile when propelled out of gun muzzle, a 2D-axisymmetric unsteady model for the combustion flow field of AP/HTPB base bleed propellant was established. The flow field characteristics in base bleed simulator under transient pressure-release were obtained through numerical simulation. Results show that in the pressure-release process, the parameters such as the pressure, the density and the temperature in the combustion chamber decrease quickly along the axial direction, the gas flow rate rises rapidly, and the gradients of various parameters gradually decrease. As a result of the base bleed propellant gas ejecting into the chamber, the density decreases along the radial direction, whereas the temperature increases on the contrary. The calculated pressure varying with time in the combustion chamber agrees well with the experimental data.