Based on combined addition technology of flame retardant and synergistic agent, the flame retardant asphalt with combined addition of flame retardant and synergistic agent was designed. Oxygen index test was conducted to study the effect of synergistic agent on flame retardancy of the flame retardant asphalt; by using synergistic flame retardant efficiency(SE) and flame retardant cost effective value(EV), synergistic flame retardant effects of the flame retardant system made with different of flame retardant and synergistic agent were investigated. And conventional asphalt performance test and dynamic shear rheological test were carried out to analyze the effect of the combined flame retardant system on the road performance of asphalt. The experimental results show that the appropriate combined flame retardant system in the proper ratio can effectively improve the flame retardancy and high temperature performance of asphalt.