Tunnel flame retardant asphalt was prepared by zinc borate（ZB） and self made flame retardant(BFR Si), and its combustion performances were systematically investigated through cone calorimetry, thermal gravimetric analysis and smoke density analysis methods. The synergistic effect of BFR Si and ZB was discussed. The results indicate that ZB and BFR Si have synergistic effect which made the smoke density(Ds), the heat release rate(HRR), mass loss rate(MLR) and the effective heat of combustion(EHC) indicators of road asphalt+BFR Si system decrease, while the ignition time become longer and the practical carbon content increases, that is to say the flame resistance performance of the flame retardant asphalt is significantly improved. Road asphalt+BFR Si system shows endothermic flame retardant mechanism and condensed phase flame retardant character, while road asphalt+BFR Si+ZB system shows the condensed phase flame retardant mechanism mainly.