AbstractTo inhibit the multiphase combustion characteristics of polymer-modified bitumen (PMB) at high temperatures, a new composite flame retardant with synergistic effects was developed based on the gradient distribution of combustion temperature ranges of four bituminous fractions: saturates, aromatics, resins, and asphaltenes (SARA). Expanded graphite (EG), ferrous hypophosphite (FHP), ammonium polyphosphate (APP), and zinc borate (ZB) were chosen to match the combustion temperature range of every bituminous fraction in turn, so that each bituminous fraction combustion behavior was inhibited by the corresponding flame retardant constituent. The optimized compounding scheme of the composite flame retardant was confirmed, and its inhibitory effects on bituminous combustion were examined using cone calorimeter tests. Test results showed that when the mixed proportion of EG, FHP, APP, and ZB was 1:3:3:4 by weight, the prepared composite flame retardant showed multiphase synergistic inhibitory effects during the entire bituminous combustion, obviously increasing bituminous flame retardancy. Simultaneously, the composite flame retardant presented greater smoke-suppressing effects during bituminous combustion. A thick pyknotic continuous carbon layer was formed on PMB after adding the composite flame retardant, and it played a flame-retarding role in the gas and condensed phases and also suppressed smoke release during bituminous combustion.