UHF-SCF-PM3 MO method has been employed to study the pyrolysis initiation reaction of tetranitrotetraazabicyclooctane in gas phase. All transition states,activation energies and potential curves of three possible reaction paths are obtained. The changes of the bond orders,geometries and atomic charges over the reaction are revealed. It is found that the homolysis of N—NO₂ bond into two radicals is the initial step of titled compound,which is similar to that of usual nitramine explosives. Finally,the thermodynamic and kinetic parameters have been investigated based on the statistical thermodynamics and transition state theory.