The invention discloses an autoclaved aerated concrete block and a preparation method thereof. The autoclaved aerated concrete block is characterized by being prepared from the following components inparts by weight: 65 to 75 parts of tailings, 10 to 15 parts of cement, 10 to 15 parts of lime, 1 to 3 parts of gypsum, 0.4 to 0.6 part of aluminite powder, 1 to 2 parts of lignocellulose, 0.2 to 0.3part of an anti-cracking agent and 60 to 70 parts of water; the anti-cracking agent is prepared by uniformly mixing the following components in parts by weight: 6 to 10 parts of diacetone acrylamide,2 to 4 parts of glazed hollow bead, 1 to 2 parts of silica powder, 0.6 to 0.8 part of hydroxypropyl methylcellulose and 0.3 to 0.5 part of bentonite. According to the preparation method disclosed by the invention, by matching the lignocellulose with the anti-cracking agent, on one hand, the anti-cracking property of aerated concrete blocks can be improved; on the other hand, water retention of theaerated concrete blocks also can be improved, and the absorption of the aerated concrete blocks to water in masonry or plastering mortar is reduced.