The invention relates to a coal fired furnace slag autoclaved aerated concrete building block and a preparation method of the coal fired furnace slag autoclaved aerated concrete building block. The concrete building block is prepared from dry materials and auxiliary materials, wherein the dry materials comprise the following ingredients in percent by weight: 35 to 60 percent of coal fired furnace slag, 20 to 35 percent of cement, 4 to 8 percent of quicklime, 0 to 4 percent of flue gas desulfurization gypsum and the balance waste materials. The waste materials are leftovers generated through cutting in the production process of the autoclaved aerated concrete building block; the auxiliary materials comprise aluminum powder paste, NaCO3, a foam stabilizing agent and a water reducing agent; the aluminum powder paste accounts for 0.08 to 0.12 percent of the weight of the dry materials; the NaCO3 accounts for 0.03 to 0.1 percent of the weight of the dry materials; the foam stabilizing agent accounts for 0.01 to 0.03 percent of the weight of the dry materials; the water reducing agent accounts for 0.03 to 0.05 percent of the weight of the dry materials. The coal fired furnace slag autoclaved aerated concrete building block has the innovation point that the coal fired furnace slag is used; in addition, the lime use amount is low; the cost is reduced; meanwhile, the autoclaved aerated concrete building block with all technical indexes capable of meeting the national standard requirements is produced.