The decomposition temperatures of composite explosives (HMX+NOG₂Tz) in different mass proportions were measured by differential scanning calorimetry (DSC). The detonation velocities of composite explosives were calculated by an empirical equation. The thermal behavior of the composite explosive 9^# (80%HMX + 20%NOG₂Tz) was studied by DSC at different heating rates. The apparent activation energy and pre-exponential factor of the exothermic decomposition reaction, the temperature (T_e0) of the extrapolated onset in heating rate β→0, and the critical temperature of thermal explosion (T_b) were calculated. The results show that the decomposition peak temperature of the composite explosive 9^# increases by 2 ℃and the apparent activation energy by 70 kJ·mol^-1over the pure HMX. It indicates that the addition of NOG₂Tz (< 50%) can improve the thermal stability of HMX.