The invention relates to a fire retardant added into an asphalt mixture and a preparation method thereof. The composite fire retardant provided by the invention consists of an organic brominated flame retardant, an inorganic synegetic flame retardant, a smoke suppressant, organic monotmorillonite, asphalt and polyethylene, wherein the adopted organic brominated flame retardant can be one or a mixture of a plurality of decabromodiphenyl ether, octabromodiphenyl ether, decabromodiphenylethane, ethylene bistetrabromophthalimide and the like; the inorganic synegetic flame retardant is flame retardant antimony trioxide or molybdenum oxide or the mixture of the flame retardant antimony trioxide and the molybdenum oxide; the organic monotmorillonite is organic modified nano-laminated silicate; the asphalt is lake asphalt or building petroleum asphalt or the mixture of the lake asphalt and the building petroleum asphalt; and the polyethylene is linear low-density polyethylene. The composite fire retardant provided by the invention has high flame retardant performance and high temperature stability and can be added into the asphalt mixture on the spot for construction.