The invention discloses a method of preparing aerated concrete by utilizing water-quenched manganese slag. Materials are as follows in percent by weight: 13%-15% of cement, 13%-17% of lime, 23%-27% of water-quenched manganese slag, 42%-44% of silica sand and 2%-3% of gypsum, wherein ratio of water to material is 0.49-0.51, and aluminum powder accounts for 0.12% of the total weight of the dry materials. The water-quenched manganese slag aerated concrete building block is completed by processes such as material stirring, casting, gas-forming, cutting, autoclaved-curing, and the like, wherein the autoclaved curing system is as follows: heating for 2.5 hours until atmospheric pressure reaches 1.2 mpa-1.25 mpa, keeping temperature and pressure constant for 6.5 hours, cooling and reducing the pressure for 2 hours to the normal temperature and the normal pressure, wherein the finally obtained product has oven-dry compressive strength of over 3.5 MPa, desiccated bulk density of 607 Kg/m<3>-615Kg/m<3>, and satisfies requirements of national B06-level aerated concrete qualified products. The method disclosed by the invention not only provide the aerated concrete products with reasonable price for the building market, but also finds a novel utilization way for water-quenched manganese slag.