The invention discloses a method for preparing aluminum oxide aerogel, and belongs to the technical field of compound preparation and industrial waste recycling. The method comprises the following steps: firstly removing impurities from aluminum slag, drying and screening the aluminum slag to obtain coarse aluminum powder; soaking the coarse aluminum powder in diluted hydrochloric acid, heating and stirring for a period of time, and performing solid-liquid separation to obtain a coarse AlCl3 solution; then adding an NaOH solution in the coarse AlCl3 solution, adjusting the pH value, and performing filtering to obtain NaAlO2 filtrate; regulating and controlling the pH value of the NaAlO2 filtrate by using dilute hydrochloric acid to obtain aluminum hydroxide colloid; and mixing the aluminum hydroxide colloid with an NaHCO3 foaming agent, and performing sintering to obtain the aluminum oxide aerogel. According to the method disclosed by the invention, the effective components in the aluminum slag are effectively used, the prepared aluminum oxide aerogel can be used as a building heat preservation material, so that the aluminum slag is used more sufficiently, and the method has the advantages of wide raw material source, small environmental pollution, low material and energy consumption, low coat and the like, thereby being suitable for industrial production.