The thermal decomposition characteristic parameters of octogen(HMX) were obtained by differential scanning calorimetry(DSC). Onset decomposition temperature(T₀), mass loss in stages(△m%), the maximum mass loss speed and corresponding temperature, final residual quantity, reaction depth, reaction rate and so on were obtained by thermogravimetry(TG). Thermal analysis data and Malek method were applied for the thermal decomposition research of HMX. The results show that HMX presents an endothermic peak at which β-HMX becomes to δ-HMX and an endothermic peak for melting in the first thermal process. Further, HMX presents a fierce exothermic peak and it′s a complete thermal decomposition process. The most probable mechanism function, critical temperature of thermal explosion and self-accelerating decomposition temperature were f(α)=(1-α)²,T_b=279.9 ℃ and T_SADT=273.3 ℃, respectively.