The invention discloses warm mixed flame-retardant asphalt. The product comprises a warm mixing agent, matrix asphalt and a flame retardant. The warm mixing agent is prepared by performing a sweating process by taking a byproduct polyethylene wax as a raw material. In the sweating process, oil soluble organic matters capable of decomposed to generate gas are pre-added into the byproduct polyethylene wax raw material, and micro-bubbles are formed in a wax layer by means of gas generated by the byproduct polyethylene wax which is heated, so that a liquid component is quickly discharged and separated; meanwhile, in the sweating process, the liquid component is carried out through the wax layer by means of the air flow to separate the solid component from the liquid component in a forced manner, so that the low molecular weight components in the raw material are removed more effectively. The warm mixed flame-retardant asphalt prepared by the method not only is good in flame-retardant effect and low in using temperature, but also can improve the thermal stability of the asphalt, improve the anti-thermal aging property, and reduce the generation amount of harmful gas in asphalt construction obviously.