The invention relates to a method for preparing blast furnace slag based foam glass by a melting method. The method comprises the steps: uniformly mixing 40-90 parts (by weight) of blast furnace slag and 10-40 parts (by weight) of quartz sand proportionally, grinding the mixture, then, putting the mixture powder into a vessel, and carrying out melting in a melting furnace for 0.5-8 hours at the temperature of 1,100-1,600 DEG C; directly casting the obtained melt in a hot mold with the temperature of 900-1,300 DEG C, introducing gas into the hot mold by dint of a gas pipe while carrying out casting molding so as to form gas holes, carrying out heat preservation for 0.5-8 hours, then, cooling the molded product to room temperature, and carrying out demolding, so as to obtain foamed glass; and carrying out cutting, sanding and polishing on the obtained foamed glass, thereby obtaining the final product. According to the method, the obtained foam glass has no need of introducing any additive, such as a foamer and a foam stabilizer, so that the production cost is reduced; and during preparation, the slag raw material is not required to subject to heated melting, water quenching, then, cooling and sintering, so that a large number of energy sources are saved, and production working procedures are simplified.