The invention discloses a method for preparing aerated concrete from water-quenched manganese slag-coal ash. According to the preparation method, the aerated concrete comprises the following components in percentage by weight: 9%-13% of cement, 11%-15% of lime, 10%-20% of water-quenched manganese slag, 52%-68% of coal ash, 2%-4% of gypsum; the ratio of water to material is 0.50-0.55; the weight of powdered aluminum paste is 0.12% of total weight of dried materials. A water-quenched manganese slag-coal ash aerated concrete building block is prepared by the procedures of raw material grinding, burdening, rapid stirring, pouring, standing gas forming, cutting and autoclaved curing. An autoclaved curing process comprises the following steps: heating for 2 hours until the temperature is 185-190 DEG C and the atmospheric pressure reaches 1.2MPa-1.25MPa, maintaining at constant temperature and pressure for 6.5-7 hours, and cooling and depressurizing for 2 hours until reaching normal pressure and temperature, so as to obtain a product, wherein the absolute dry compressive strength reaches more than or equal to 2.55MPa, and the dry volume density is 478kg/m<3>-525kg/m<3>. According to the high-quality B05-level aerated concrete building block product prepared by virtue of the method, a light and high-strength aerated concrete building block product is provided for a building material market, and a new path is found for the resource utilization of the water-quenched manganese slag.