The thermal decomposition process of HMX was studied by dynamic vacuum stability test (DVST) method. The activation energy and pre-exponential constant of thermal decomposition reaction of HMX were obtained by universal integral method and differential method. The results show that the mechanism functions of thermal decomposition reaction of HMX at different isothermal temperatures are different. The thermal decomposition process of HMX obeys Ginstling-Brounshtein equation with 3D mechanism in the temperature range of 100-110 ℃, parabola rule with 1D mechanism in the temperature range of 120-140 ℃ and Valensi equation with 2D mechanism at 150 ℃. Reaction rate constant (k) of the thermal decompositionreaction of HMX increases with temperature, but it does not accord with the Van′t Hoff rule. The amount of decomposition gases of HMX below 150 ℃ obtained by DVST are less than 2.0 mL·g^-1, showing that HMX has a good thermal stability. The Semenov equation obtained by the time(t) required to decompose 0.1% HMX at different temperature(T) is lnt_T=12157.95/T-19.0052. The effective storage life needed to consume 0.1% HMX at 25 ℃ deduced by this equation is 90.6 a.