The quantum-chemical UHF-SCF-PM3 MO method has been employed to calculate the pyrolysis initiation reactions of α、β、γ、ε and δ polymorphs of hexanitrohexaazaisowurtzitane in gas phase. Their transition states,activation energies,and potential energy curves have been obtained. The changes of the geometries,energies and atomic charges over these reactions are revealed. It is found that the mechanism of pyrolysis initiation reaction of titled compound is similar to that of usual non-caged nitramine explosives. In addition,the relationship between the activation energy and the impact sensitivity is also discussed.