The invention discloses a preparation method of a flame-retardant material, and relates to the field of building materials, in particular to the field of flame-retardant materials for buildings. The flame-retardant material is prepared by high-temperature-resistant silica gel, ceramic powder and a hardening agent through twice high-temperature sintering. The flame-retardant material is mainly made of inorganic materials, has a noncombustible property, and the weight is light, so that the load on a steel structure is not increased, and meanwhile, the structure of the flame-retardant material is compact and impact-resistant after high-temperature sintering, and the duration of fire resistant is more than 3 hours according to the proofness of fire resistance experiments.