The whole process of thermal decomposition of (2,2′,4,4′,6,6′)hexanitro-stilbene (HNS ) is investigated by using TG-DTG,TG-MS and in-situ thermolysis rapid scan FTIR coupling techniques.Kinetic parameters of the thermal decomposition of HNS are obtained and a possible decomposition mechanism is suggested. Results show that the thermal decomposition process can be divided into two stages: (1) forming "explosive coke" via the nitro-nitrite isomerization on the C-NO₂ p-substituted to α-CH bond and the decomposition of "five cycled compound" intermediate between α-CH and o-nitrio in the first stage, (2) releasing CO₂ and other gases via "explosive coke" decomposing in the second stage.