The transient combustion characteristics of ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) base bleed propellant, associated with the transient instability by sudden depressurization in the moment that the base bleed projectile flies away from the muzzle, was investigated experimentally. The combustion pressure is between 3.15 MPa and 7.24 MPa and the depressurization rate is controlled in the range of 13.2-136.7 MPa/s. The gas phase flame blows away from the nozzle, leading to a thicker gas phase reaction layer and smaller heat feedback to the propellant under rapid depressurization. Two main traits were presented under depressurization, which is gas phase flame extinction and non-extinction. There is a smallest critical depressurization rate to extinguish AP/HTPB base bleed propellant, which increases as the maximum combustion pressure increases and has the empirical relationship of vertical bar dp/dt vertical bar(cr) = 58.29 p - 140.78 MPa/s. Although the gas phase flame is extinguished by rapid depressurization, the condensed phase decomposition may still exit and a stable re-ignition flame may occur if enough heat is accommodated. The gas phase flame extinction behaviors under transient depressurization can be divided into three regions, which are extinction with then re-ignition, extinction with maintaining decomposition, and absolutely extinction.