Al powder was stored in saturated water vapor, oxygen, nitrogen and drying air separately for a time period of up to six months, the degradation behavior of Al activity was characterized by the reaction of Al with water. It was found that water vapor decreased the induction time for the beginning of Al-water reaction and reduced the total hydrogen generation per unit weight of Al, while oxygen increased the induction time and retarded the Al-water reaction. In contrast, the effect of nitrogen and drying air on Al activity was weak. The mechanism analyses indicated that water vapor promoted the hydration of the Al surface passive oxide film and sped up the reaction of Al with water, while oxygen thickened the passive oxide film of the Al surface and prolonged its hydration process. These results imply that water vapor rather than oxygen is responsible for the degradation of Al activity during storage in the atmospheric environment.