The invention provides a magnesite slag aerating brick. The magnesite slag aerating brick is processed and manufactured by the following raw materials in parts by weight: 20 to 30 parts of magnesite slag, 25 to 30 parts of fly ash, 5 to 10 parts of cement, 4 to 6 parts of modified kaolin, 5 to 10 parts of sewage treatment sludge, 5 to 10 parts of caustic sludge, 5 to 10 parts of calcium carbonate powder, 10 to 15 parts of lime, 0.1 to 0.3% of triethanolamine based on weight of lime, 4 to 5 parts of gypsum, 0.02 to 0.08 part of aluminum powder, 3 to 5% of oxyparaffin soap based on weight of aluminum powder, 0.01 to 0.02 part of calcium bicarbonate, 2 to 3 parts of expanded vermiculite, 1 to 2 parts of lignocellulose, 0.3 to 0.5 part of water glass, 0.03 to 0.05 part of sodium borate, 1 to 2 parts of sodium hexametaphosphate, 1 to 2 parts of hydroxypropyl methyl cellulose sodium and 1 to 2 parts of superfine polyvinyl alcohol. According to the magnesite slag aerating brick and the preparation method provided by the invention, the magnesite slag is added to the raw material of the aerating brick, and other common assistants and modified nanometer additives are synchronously added; the aerating brick processed by a proper technology is low in cost and high in performance, the circulating economic benefit of turning waste into wealth is achieved, and the pollution is reduced; the processed aerating brick has the characteristics of being light in unit weight, high in heat insulating performance, high in effect of sound absorbing and high in strength; and the aerating brick can be used as the non-bearing wall or bearing wall.