The invention discloses a lost foam casting cast iron coating for preventing carbon defects, which comprises the following components by weight percent: 60.5%-64.5% of bauxite, 13.5-17.5% of kaolin, 3.5%-6.0% of talc powder, 1.7%-3.5% of attapulgite, 0.4%-0.8% of sodium lignosulphonate, 2.5%-6.0% of aluminum sulfate, 1.7%-3.5% of polyvinyl alcohol, 3.5%-5.0% of silica sol, 0.1%-0.2% of iron oxide powder and 0.01%-0.02% of n-octanol, and the sum of the components is 100%. The lost foam casting cast iron coating is prepared by respectively preparing water solution of the attapulgite, the polyvinyl alcohol and the aluminum sulfate, evenly mixing the bauxite, the kaolin, the talc powder and the iron oxide powder for preparing mixed powder material, mixing the water solution of the attapulgite, the polyvinyl alcohol and the aluminum sulfate, the silica sol and the refractory mixed powder material with the n-octanol, stirring and grinding. The coating has the advantages of good thixotropy, good coating property, high coating strength and good gas permeability; and the surface of a cast is smooth and has no wrinkles and other carbon defects.