The invention relates to a method for extracting aluminum oxide from fly ash through sulfuric acid activation impurity removal. The method comprises the steps of: adding sulfuric acid into fly ash; performing heating activation for dissolution out; dividing a dissolved-out solution containing aluminum into two parts; adding sodium hydroxide into one part (3/4) of the solution to generate sodium metaaluminate, so as to remove iron, magnesium, calcium and the like; adding an organic precipitant HTP-19 into the other part (1/4) of the solution to remove iron ions in the solution; then neutralizing the two parts of filtrates to generate aluminum hydroxide; calcining the aluminum hydroxide to generate aluminum oxide; performing causticization on sodium sulfate filtrate to generate sodium hydroxide and calcium sulfate; performing double decomposition on the calcium sulfate and ammonium carbonate to generate ammonium sulfate and calcium carbonate; performing catalytic decomposition on the ammonium sulfate to obtain ammonia gas and sulfur trioxide; sintering the calcium carbonate to obtain calcium oxide and carbon dioxide; and synthesizing the ammonia gas and the carbon dioxide into ammonium carbonate (for recycling). According to the method, full recycling of materials is realized, the process energy consumption and the cost are significantly reduced, the extraction rate of aluminum oxide reaches 93.6-95.8%, and the purity of the produced aluminum oxide is more than 98.9%.