The invention discloses a method for producing aluminum oxide by a Bayer process and relates to improvement of increasing slurry concentration with back feeding bauxite digestion technology during an aluminum oxide production process. The method is characterized by comprising the following steps: adding gibbsite-type bauxite to a flash evaporation process; increasing Rp of the digested bauxite slurry after the convergence to 1.30-1.35; adding decomposition mother liquor or flat plate washing liquor with Rp of 0.10-0.60 in the dilution process; reducing the Rp of the crude liquor to 1.15-1.18; and refining the crude liquor. In the method provided by the invention, a method of adding alkali after bauxite sweetening is adopted, thus the solid content of back feeding bauxite is effectively increased; and as the alkali liquor is added after the digestion, so that the amount of the alkali liquor involved in the back feeding bauxite digestion is remarkably reduced, and the bauxite slurry reduced ratio of a back feeding bauxite system is effectively reduced, thus the steam consumption of the back feeding bauxite digestion is reduced. Through the invention, the capacity of a single system of the Bayer process can be further increased, a system of the Bayer process is maximized, the power consumption and the evaporation steam consumption are reduced, and the high-quality operation of the system is maintained; and besides, the sustainable stable operation of the high-yield situation is ensured while the system capacity is increased.