The fast thermolysis processes of ammonium perchlorate (AP) and its chemical reaction mechanism under certain simulated conditions of combustion were systematically studied by means of T-Jump/FTIR technique. AP was flash-pyrolyzed under different pressure nitrogen atmosphere, with the heating rate of 1 000 K·s-1 and the set temperatures of 874 and 1 274 K. Thereafter, the real time species and concentration of its gaseous products were obtained and identified by using rapid scanning Fourier transform infrared (FTIR) in-situ spectroscopy. It was indicated that the main gaseous products of AP thermolysis is composed of NO2, N2O, NO, HCl and NClO. Interestingly, the values of N2O/NO2, NO/NO2 and NO/NClO are increased while the test temperature or pressure is elevated. It is suggested that the transformation of condensed phase and heterogeneous gas/condensed phase was probably arises and the “following reactions” among the main gaseous products may also take place in the fast thermolysis process of AP.