The invention discloses foam glass prepared from fly ash as well as a preparation method and application of the foam glass, and belongs to the technical field of foam glass preparation. Fly ash and glass powder are adopted as raw materials and mixed with a foaming agent, then a PVB alcoholic solution is added for granulation, and after molding, high-temperature foaming is conducted through two times of heating, so that the foam glass is obtained. Through two times of heating and heat preservation, part of dioxin can be effectively decomposed, and meanwhile, the formation amount of a high-temperature melt phase is promoted so as to effectively coat residual dioxin, high chlorine, high calcium and heavy metal components. According to the method, the high-temperature melt effectively coats the high chlorine, high calcium, heavy metal components and residual dioxin, the heavy metal elements are left to stand in water for a long time, leakage of the high calcium and high chlorine componentsis little, and harm of the components to the environment is avoided. The method is simple in process and suitable for large-scale production, the fly ash is adopted as the raw material, and the problem of environmental pollution is solved while the low-cost raw material is obtained.