The invention relates to a sulfuric acid hot leaching based method for extracting aluminum oxide from fly ash. The method comprises steps as follows: sulfuric acid is added to the fly ash, a mixture is heated, activated and subjected to dissolving out, alkali is added for precipitation, alkali is added to a filter cake after solid-phase separation, a sodium metaaluminate solution is formed, filtering and impurity removing are performed, aluminum hydroxide is generated through sodium metaaluminate seed precipitation, and metallurgical-grade aluminum oxide is generated through calcination; a filtrate, namely, a sodium sulfate solution, is causticized to generate sodium hydroxide and calcium sulfate, calcium sulfate and ammonium carbonate are subjected to double decomposition to generate ammonium sulfate and calcium carbonate, ammonium sulfate is catalyzed and decomposed into ammonia gas and sulfur trioxide, calcium carbonate is burnt and decomposed into calcium oxide and carbon dioxide, and ammonium carbonate is synthesized by ammonia gas and carbon dioxide; the materials are all recycled, the energy consumption in the process is low, and sulfur trioxide is subjected to water absorption to generate sulfuric acid. According to the method, a feasible technological route is built, reducing utilization of the fly ash is realized, the materials are all recycled, technological energy consumption and the cost are remarkably reduced, and the aluminum oxide extraction ratio reaches 91.6%-94.7%.