The invention provides a method for extracting alumina by mixed calcination of ammonium sulfate and fly ash under a reducing atmosphere, and belongs to the technical field of comprehensive utilizationof fly ash, wherein fly ash and an appropriate amount of ammonium sulfate are mixed to prepare a raw material, the raw material and an appropriate amount of a reducing agent are calcined to obtain acalcined clinker, and the calcined clinker is used for extracting alumina. According to the present invention, with the method, the ammonia degradation during the mixed calcination of ammonium sulfateand fly ash is avoided so as to increase the utilization rate of ammonium sulfate; and the calcined clinker is further subjected to reducing thermal decomposition, and the metallurgical grade aluminais produced by using the low-temperature Bayer methd, such that the complex processes such as aluminum sulfate leaching, solid-liquid separation, aluminum sulfate solution purification, crude aluminum hydroxide precipitation, crude aluminum hydroxide washing, ammonium sulfate concentration crystallization and the like in the existing ammonium process are eliminated, the ammonium sulfate consumption is low, the process is short, the energy consumption is low, and the alumina production cost is low.