The method of thermogravimetry/differential scanning calorimetry–mass spectrometry–Fourier transform infrared (TG/DSC-MS-FTIR) simultaneous analysis has been used to study thermal decomposition of ammonium perchlorate (AP). The processing of nonisothermal data at various heating rates was performed using NETZSCH Thermokinetics. The MS-FTIR spectra showed that N₂O and NO₂ were the main gaseous products of the thermal decomposition of AP, and there was a competition between the formation reaction of N₂O and that of NO₂ during the process with an iso-concentration point of N₂O and NO₂. The dependence of the activation energy calculated by Friedman's iso-conversional method on the degree of conversion indicated that the AP decomposition process can be divided into three stages, which are autocatalytic, low-temperature diffusion and high-temperature, stable-phase reaction. The corresponding kinetic parameters were determined by multivariate nonlinear regression and the mechanism of the AP decomposition process was proposed.