The invention discloses a method for extracting alumina from fly ash. The method comprises the following steps of: a), acid treatment, i.e. mixing and feeding hydrochloric acid and the fly ash into a digester lined with a polyfluortetraethylene material for dissolving, carrying out separation on dissolved slurry by a settling tank, carrying out evaporation, crystallization, roasting and decomposition on the upper overflow slurry to form active initial alumina and hydrogen chloride gas, and returning the hydrogen chloride gas to a fly ash dissolving process after the hydrogen chloride gas is recovered to prepare acid; and b), Bayer process treatment, i.e. mixing the active initial alumina and circulating mother liquor, sending the mixture into an autoclave for dissolving, carrying out separation on the dissolved slurry by the settling tank, carrying out fine filtering on the upper overflow slurry to obtain refined fluid, mixing the refined fluid and seed crystals of aluminium hydroxide, carrying out seed crystal digestion to obtain decomposition mother liquor and aluminium hydroxide, returning the decomposition mother liquor to the process of dissolving the active initial alumina after the decomposition mother liquor is evaporated, and carrying out filtering, washing and roasting on the aluminium hydroxide to obtain an alumina product. Due to the adoption of the method, the metallurgical alumina which meets the national standard requirements can be prepared.