Previous works indicated that aluminum hydroxide could be acted as catalyst to speed up the hydrolysis reaction of Al, and the catalytic activity of aluminum hydroxide is related to its microscopic properties. In this work, the effect of crystalline phases of aluminum hydroxide catalysts on Al hydrolysis is researched systematically. The results show that the hydrogen production performance of Al hydrolysis catalyzed by aluminum hydroxide with different crystalline phases is obviously different, and the catalytic activity of different aluminum hydroxides is in the order of boehmite > bayerite > gibbsite. A possible mechanism is given, which reveals that the surface hydroxyl groups of aluminum hydroxide act as anchoring sites for the adsorption of H2O molecules and decrease the dissociative adsorption energy of H2O molecules on aluminum hydroxides surface, promoting the hydrolysis reaction of Al. Consequently, the difference in catalytic activity of aluminum hydroxide with different crystalline phases is attributed to the difference in surface hydroxyl group density. This study provides a new idea for the synthesization of aluminum hydroxide with excellent catalytic activity.