The invention belongs to the field of building materials, and discloses an autoclaved aerated concrete block prepared by using steel slag, fly ash and cement as main raw materials. The autoclaved aerated concrete block is prepared by the following steps: mixing the main raw materials such as the steel slag, the fly ash, the cement and lime and water, performing uniform stirring, adding aluminum powder, performing foaming under stirring, pouring the foamed material into a mold, performing molding under steam curing, performing demoulding, continuing curing under steam curing conditions for 20-24 h, and finally performing autoclaved curing in an autoclave to prepare the autoclaved aerated concrete block meeting a national standard GB11968-2006 standard in China. The autoclaved aerated concrete block provided by the invention has the advantages that the raw materials are simple and economical, the process is simple to operate, and the problem of accumulation of a large amount of the steelslag can be solved at the same time.