In order to study the out-of-phase blowing effect of ammonium perchlorate/hydroxyl terminated polybutadiene (AP/HTPB) under transient depressurization, a two-dimensional periodic sandwich model of AP/HTPB sandwich unsteady combustion was established. The gas-solid two-step total reaction was used to couple the gas-solid boundary layer, and the AP/HTPB step-down temperature criterion was applied. The numerical simulation comparative analysis under the initial combustion pressure 3.5MPa approximate to 10MPa and the initial depressurization rate of 1000MPa/s approximate to 2000MPa/s was experimented. The results show that under the condition of initial combustion pressure of 3.5MPa and pressure reduction rate of 1000MPa/s, the formation of narrow diffusion chemical reaction zone appears in the initial stage of depressurization; when the combustion pressure drops to about 1.7MPa, the flame appears dual characteristics: diffusion and premixed combustion; when the pressure dropped to 0.1MPa, the flame is premixed combustion. AP(g) is the main factor causing the out-of-phase blowing effect. During the initial stage of combustion, AP(g) decomposes rapidly and its decomposition product is close to the combustion surface; with the rapid decrease of pressure, the gas-phase heat feedback decreases, leading to the decrease of AP(g) decomposition rate and AP(g) diffusion to the gas phase, which is the initial phase of out-of-phase blowing effect; when the gas phase heat feedback reduced and the temperature is below 750K, the decomposition of AP(g) is stopped and the undecomposed AP(g) is further diffused to the gas phase region, which exacerbates the out-of-phase blowing effect, leading to the extinguishment of AP/HTPB. The unsteady combustion flame extinction time increases with the initial combustion pressure, and decreases with the initial depressurization rate.