The influence of nanometer-Al₂O₃ on the impact sensitivity of HMX/nanometer-Al₂O₃ was studied through testing the drop hammer impact of HMX and HMX/nanometer-Al₂O₃. The action mechanism of the nanometer-Al₂O₃ in the composite explosive was also investigated. The results indicate that the nanometer-Al₂O₃ has lubrication role. The impact sensitivity of HMX/nanometer-Al₂O₃ reduces obviously as increasing the nanometer-Al₂O₃ content in the composite explosive. The impact sensitivity of HMX/nanometer-Al₂O₃(98/2) is higher 13.1 cm than that of HMX using 2.5 kg drop hammer. The results can provide a valid approach for the improvement of military composite explosive impact safety.