The thermal decomposition of hexanitrohexaazaisowurtzitane (HNIW) was studied by In-situ FTIR spectroscopy. The spectra of condensed-phase reactant and its decomposition products were recorded from room temperature to 450 ℃. The absorption bands of the condensed-phase decomposition products were assigned. The results of the subtraction FTIR spectra show that a nitro group on six-membered ring homolyzes in the range of the crystal transformation temparature 110 ℃ to 150 ℃. This homolysis causes a series of cleavage reactions to form compounds contained α,β-unsaturated aldehyde and α,β-unsaturated nitroso-group. Above 155 ℃, a nitro group on five-membered ring begins to homolyze and form five-membered ring lactam. Above 165 ℃, more cleavage reactions and self-oxidations occur. Above 190 ℃,oxidation reactions between product molecules dominantly happens besides self-oxidations. A lot of decomposition products are formed,such as dioxoimidazole,dioxopiperazine,acid anhydride,cyanuric acid,ammonium,four,five and six-membered ring lactam etc. Carbodiimide appeared in residues at temperatures higher than 300 ℃ is attributed to the decomposition of amine compounds. Above 400 ℃,chief components in the residue are melem and its polymers such as melon etc.