The geometries of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) and its complexes with the pyrolysis products (NH₃,ClO₃) of ammonium perchlorate (AP) were optimized using density function theory (DFT) at the B3LYP/6-31g(d) level. The bond dissociation energies for N—NO₂ in β-HMX and its complexes at normal, 4 MPa and 6 MPa pressures were obtained. Results show that the geometrical configuration of HMX complexes with NH₃ and ClO₃ are similar to β-HMX structure. It indicates that, the equatorial nitryls of NH₃—,ClO₃— complexes are also prior to the axial nitryls in its pyrolysis process and there are trivial changes on the N—NO2 bond dissociation energies when β-HMX combines with NH₃ or ClO₃. However, once the complexes decompose, the product NO₂ can react with NH₃ more easily. This exothermic reaction may induce the subsequent pyrolysis process of HMX. The high pressure has no influence on the effects of NH₃ and ClO₃ on the initial pyrolysis mechanism of β-HMX.