Two simultaneous devices of the fast thermlysis reaction cell (gas reaction cell) in situ in conjunction with rapid-scan fourier transform infrared spectroscopy (RSFT-IR), and the solid reaction cell in situ /RSFT-IR were used to measure the gas products and condensed phase products of the thermal decomposition of HMX/RDX-NEPE propellant in real time. Under linear temperature increase condition, the thermal decomposition characteristics of HMX/RDX-NEPE propellant was studied. On the basis of the results obtained, the thermal decomposition mechanism was discussed.