A method for manufacturing a foam material by using a molten slag, comprising: introducing the molten slag into a pool for preserving heat and modifying, wherein a temperature of the molten slag is maintained at 1400°C-1500°C, and adding a viscosity modifier and optionally a color modifier to the molten slag to adjust a viscosity and optionally a color of the molten slag according to requirements of a product to be manufactured; introducing the molten slag discharged from the pool for preserving heat and modifying into a foaming pool while adding a foaming agent to the molten slag, wherein the molten slag in the foaming pool is controlled at 1250°C-1400°C so as to foam and mold; and maintaining the foamed and molded slag at 800°C-1000°C for 20-30 minutes in a non-reducing atmosphere, and then naturally cooling the foamed and molded slag to an ambient temperature so as to obtain the foam material, wherein the molten slag includes 10-40wt% of Al 2 O 3, 5-25wt% of MgO, 10-50wt% of SiO 2, 10-40wt% of CaO, 0.1-5wt% of TiO 2, 0.1-5wt% of FeO and 0.1-5wt% of MnO, wherein the viscosity modifier is at least one of clay, porcelain clay, lodestone, pottery clay, feldspar, quartz sand and TiO 2, and the viscosity modifier is added in an amount of 3-10wt% based on a weight of the molten slag, wherein the color modifier is at least one of oxides of Ti, Cr, Ni, Cu, Co and Fe, ore powders containing the oxides and industry wastes containing the oxides, and the color modifier is added in an amount of 0-5wt% based on the weight of the molten slag, and wherein the foaming agent is carbonate and is added in an amount of 3-5wt% based on the weight of the molten slag, and the carbonate includes at least one of calcium carbonate, magnesium carbonate, calcium magnesium carbonate, other minerals containing carbonates and a mixture thereof. introducing the molten slag into a pool for preserving heat and modifying, wherein a temperature of the molten slag is maintained at 1400°C-1500°C, and adding a viscosity modifier and optionally a color modifier to the molten slag to adjust a viscosity and optionally a color of the molten slag according to requirements of a product to be manufactured; introducing the molten slag discharged from the pool for preserving heat and modifying into a foaming pool while adding a foaming agent to the molten slag, wherein the molten slag in the foaming pool is controlled at 1250°C-1400°C so as to foam and mold; and maintaining the foamed and molded slag at 800°C-1000°C for 20-30 minutes in a non-reducing atmosphere, and then naturally cooling the foamed and molded slag to an ambient temperature so as to obtain the foam material, wherein the molten slag includes 10-40wt% of Al 2 O 3, 5-25wt% of MgO, 10-50wt% of SiO 2, 10-40wt% of CaO, 0.1-5wt% of TiO 2, 0.1-5wt% of FeO and 0.1-5wt% of MnO, wherein the viscosity modifier is at least one of clay, porcelain clay, lodestone, pottery clay, feldspar, quartz sand and TiO 2, and the viscosity modifier is added in an amount of 3-10wt% based on a weight of the molten slag, wherein the color modifier is at least one of oxides of Ti, Cr, Ni, Cu, Co and Fe, ore powders containing the oxides and industry wastes containing the oxides, and the color modifier is added in an amount of 0-5wt% based on the weight of the molten slag, and wherein the foaming agent is carbonate and is added in an amount of 3-5wt% based on the weight of the molten slag, and the carbonate includes at least one of calcium carbonate, magnesium carbonate, calcium magnesium carbonate, other minerals containing carbonates and a mixture thereof.