As the base-bleed projectile flies out of the muzzle, the environmental pressure in the base-bleed combustion chamber suddenly decreases and AP/HTPB base-bleed propellant suffers intense unsteady combustion. To further study the unsteady combustion characteristics of base-bleed propellants, a semi-closed bomb as was designed experimental device and transient depressurization conditions of the muzzle were simulated. Measurements of the transient combustion characteristics of the base-bleed propellant under high depressurization rate were carried out by using a high speed digital camera system. In the experiments, the combustion chamber pressure of the semi-closed bomb was controlled from 20 to 90 MPa and the depressurization rate was controlled from 400 to1.12 x 10(4) MPas(-1). The experimental results indicate that, the out-of-phase blowing effect is intense under rapid depressurization condition, leading to the reaction layer thickened. The thermal feedback to the solid surface decreases and thus the combustion reaction of gas phase is so difficult to maintain that it begins to extinguish. However, the thermal decomposition of the solid phase is still continuing and a yellow fog can be observed above the combustion chamber nozzle. Depending on the maximum pressure in the combustion chamber and depressurization rate, the transient combustion behavior of AP/HTPB base-bleed propellant displays three patterns, i.e., automatic reignition, oscillating combustion (a critical type) and permanent extinguishment. Three unsteady combustion behaviors are preliminarily analyzed based on the thermal feedback. If the initial pressure in the combustion chamber before depressurization is larger or the depressurization rate is smaller, the base-bleed propellant tends to automatically reignite earlier and the combustion process is more stable.