The invention discloses a method for manufacturing slag fiber by using thermal-state blast furnace slag, and belongs to the technical field of resource environment. The method comprises the steps of: adding the other reactant containing acid oxides into the thermal-state blast furnace slag, melting the mixture for a reaction, and obtaining the slag fiber by a set fiber forming method after the reaction. The other reactant is pure acid oxides or minerals or industrial wastes containing the acid oxides. The acid oxides comprise silicon dioxide and/or aluminum oxide. The weight ratio of the thermal-state blast furnace slag to the acid oxides is 40-95: 5-60. The method can be used for combined production of steel and power industries to achieve the reasonable utilization of industrial raw materials and waste energy, so that the method has important significance. In addition, the slag fiber manufactured by the method can be used for various heat preserving materials and heat insulating materials; and by adding special additives, inorganic slag fiber with different performance can be obtained, and used in wider fields such as reinforcing materials, textile materials, insulating materials and the like.