Thermal decomposition and kinetics of 1-methyl-2, 4, 5-trinitroimidazole(MTNI) were studied by using TG-DTG, TG-FITR, TG-MS and RCFT-IR in situ thermolysis cell or fast thermolysis probe with rapid-scan Fourier transform infrared spectroscopy(thermolysis/RSFT-IR and fast thermolysis/RSFT-IR) methods, respectively. Results show that the exothermic decomposition process can be derived into two steps: the rupture of the O—NO bond and ring breaking. The main products of thermal decomposition detected by TG-MS method were NO₂, CO, CO₂ and H₂O. On the basis of experimental results, the thermal decomposition mechanism of MTNI is outlined.