B3LYP/6-31G(d, p), MP4(SDTQ)/6-31G(d, p) and G3MP2B3 methods were used to calculate the energy of the decomposition species of FOX-7. Based on the energy obtained by G3MP2B3 method, the rate constants of C—NO₂ cleavage and nitro-to-nitrite rearrangement were calculated in the temperature range of 250-3300 K. It is found that C—NO₂ cleavage is the dominant initial thermal decomposition step at high temperature which is consistent with the conclusion based on ab initio molecular dynamics simulation. The effects of NO₂ on the FOX-7 decomposition were investigated. Results show that the decomposition energy barrier becomes lower when NO₂ reacts with FOX-7.