The invention provides a method for preparing alumina. The crystal structure of alumina belongs to gamma-alumina, the surface area is 220-450m<2>/g, the pore volume is 0.2-0.5 cm<3>/g, and alumina hasconcentrated bimodal pore size distribution. The preparation method comprises the following steps: placing aluminum sulfate in a reactor, introducing gas into the reactor, and then performing heatingto a set temperature by a certain heating program for constant temperature treatment for a certain time to obtain alumina. The preparation method is simple and implementable, and the production costis low.