The thermal decomposition of HTPB/AP composite propellants and the influence of pressure and Al powder on the thermal decomposition were investigated by PDSC, TG-DTG and simultaneous DSC-FTIR. The results show that the thermal decomposition of AP component in HTPB/AP is delayed due to the coating deactivation of HTPB on AP. The thermal decomposition of HTPB component is accelerated with AP. The thermal decomposition of HTPB/AP system can be accelerated by both the pressure and Al, and the burning rate is increased as well. The pressure causes a phenomenon of multiple peaks which appear on DSC curves of the thermal decomposition for HTPB component, but Al can restrain the phenomenon. Furthermore, the “after-solidify” process of HTPB result from AP component and the pressure strengthens this process.