The invention discloses a preparation method of anhydrous sodium sulfide. The preparation method comprises the following steps: with industrial waste gas such as sulfuric acid, sulfur dye, etc. as a source of hydrogen sulfide gas, reacting the industrial waste gas after absorption and purification treatment with an excessive alkaline solution to obtain liquid sodium sulfide, and performing crystallization and vacuum drying treatment to obtain the anhydrous sodium sulfide. According to the preparation method, the technical route of directly reacting the alkaline solution with hydrogen sulfide gas to obtain the sodium sulfide is adopted, so that the consumption of a great quantity of non-renewable resources and the production of a great quantity of industrial waste residues and waste gas are completely avoided; the industrial waste gas such as sulfuric acid, sulfur dye, etc. is taken as the source of hydrogen sulfide gas, so that the production cost in industrial preparation of hydrogen sulfide gas is saved; and meanwhile, the anhydrous sodium sulfide is prepared by combining freezing crystallization or evaporative crystallization with vacuum drying, and crystallization mother liquor is recycled and is further concentrated and crystallized repeatedly, so that not only produced waste liquid is reasonably utilized, but also the product quality can be effectively improved. Therefore, the preparation method has a great practical value and industry prospect.