The invention discloses a method for preparing high-activity aluminum oxide powder using aluminum ash, and belongs to the preparation field of aluminum oxide powder. The method comprises the following steps: by using aluminum ash as a raw material, floating in water to remove slag, adding sulfuric acid solution and diisopropanolamine to remove insoluble matters in the aluminum ash, and then removing other metal impurities soluble in potassium hydroxide under the effect of carbon disulfide assisted by titanium dioxide, adding polyethylene glycol and ammonia water, stirring, filtering to obtain jelly, washing, drying, calcinating, and crushing to obtain the high-activity aluminum oxide powder. The method disclosed by the invention has the beneficial effects that aluminum ash is sufficiently utilized to prepare the high activity aluminum oxide powder, not only the environment pollution problem is solved, but also the sustainable utilization of the resource is realized; the prepared aluminum oxide powder is small in grain, uniform in grain size distribution and hard to unite, and the activity is more than or equal to 18% higher than that of the aluminum oxide powder prepared by other methods.