The invention discloses a method for extracting aluminum oxide from fly ash. The method comprises the following steps of: a, acid-method treatment: mixing hydrochloric acid and fly ash, and transferring the mixture in a digester, of which the lining is made of polytetrafluoroethylene materials, for digestion, performing the separation of the pulp digested in a settling tank, evaporating and crystallizing upper-part overflow, baking the crystals to decompose the crystals into active coarse aluminum oxide and hydrogen chloride gas, wherein the hydrogen chloride gas is recovered to make acid and the acid is returned to the fly ash settling step; and b, bayer method treatment: mixing the active coarse aluminum oxide and circulating mother liquor, transferring the mixture in the digester for digestion, separating the pulp digested by settlement, performing refined filtration of the upper-part overflow to obtain refined solution, mixing the refined solution and aluminum hydroxide crystal seeds, decomposing the crystal seeds to obtain decomposed mother liquor and aluminum hydroxide, and filtering, washing and baking the aluminum hydroxide to obtain the aluminum oxide product, wherein the decomposed mother liquor is evaporated and then returned to the step of digesting the active coarse aluminum oxide. By adopting the method of the invention, the metallurgical-grade aluminum oxide meeting the requirements on national standards can be prepared.