The invention relates to a method for extracting alumina from fly ash based on an ammonium sulfate method. The prior art utilizing fluidized bed-based roasting activation of fly ash to realize alumina extraction produces the problems of material caking and adhesion to the wall so that industrialization is realized difficultly and crude alumina produced by the prior art needs resolution impurity-removal so that processes are more, efficiency is low and energy consumption is high. The method comprises that fly ash, ammonium sulfate and a propping agent are mixed, the mixture is subjected to granulation, the granules are subjected to two-section fluidized bed roasting, the clinker is heated and dissolved by water, the filtrate is subjected to combined impurity-removal and then ammonia precipitation crystallization, and the crystals are calcined to form alumina. The method utilizes mixing granulation of fly ash, ammonium sulfate and the propping agent, oxygen-rich two-section fluidized bed roasting and heating dissolution by water, changes the order of the impurity-removal processes, utilizes a technical scheme comprising impurity-removal and then ammonia precipitation crystallization for high-purity aluminum hydroxide production to solve the problems of equipment corrosion, roasting-caused adhesion to the wall, clustering and coking of the prior art, shortens the integral main process and greatly reduces energy consumption and a cost of extraction of alumina from fly ash.