The invention discloses a method for extracting aluminum oxide from coal gangue, which comprises the following steps of: mixing and grinding the broken coal gangue and sodium carbonate in a certain ratio, roasting, performing circular extraction, precipitating, filtering, decomposing carbon, recycling carbon dioxide and an auxiliary agent, washing, calcining and the like to extract high-purity aluminum oxide from fly ash. Compared with other methods for extracting aluminum hydroxide from the fly ash, the method has the advantages that: the extraction ratio of the aluminum hydroxide is high, residues are a few, energy consumption is low, the process is simple, cost is low, the generated CO2 and the used auxiliary agent raw material can be recycled, and the like; and the problems of a complicated technology and high cost of the conventional process for extracting the aluminum hydroxide from the fly ash are solved, and a new way is opened up for the high-added value development and utilization of the coal gangue.