The invention relates to a preparation method of a foamed concrete block. According to the preparation method, the technical problems that the adding proportion of industrial solid is slightly low and a high-quality product with high strength and low density is difficult to be prepared in existing preparation methods of the foamed concrete blocks are solved. According to the technical scheme, the preparation method comprises the following steps: compounding superfine powder of multiple industrial solid wastes according to a formula, adding water into mixed powder, and simultaneously stirring to prepare slurry; adding a certain amount of foaming agents into the slurry, and continuing to uniformly stir, so as to prepare formed slurry; finally, pouring the formed slurry into a cavity of a forming mold, and sequentially arranging a sealed ventilating layer and loads to the upper surface of the slurry in the mold; and after foaming and shaping, carrying out demolding and cutting, and finally, carrying out autoclaved curing, so as to obtain a foamed concrete block product. The foamed concrete block is prepared by virtue of a load pressurizing manner, so that the preparation method is beneficial to preparation of the high-strength and low-density foamed concrete block, simple in operation and suitable for industrial production.