Abstract: To effectively solve the problems of large amount of modifier, poor thermal storage stability, and thermodynamic instability faced by the current production of polymer modified asphalt, the polyurethane-precursor-based reactive modifier (PRM) and styrene-butadiene-styrene (SBS) were employed to prepare the PRM/SBS composite modified asphalt in this study. Taking base asphalt, PRM modified asphalt and SBS modified asphalt as references, the performance and modification mechanism of PRM/SBS composite modified asphalt were studied by macro-micro methods. Results show that the composite modification process presents both physical and chemical characteristics. PRM can not only produce obvious chemical cross-linking with asphalt molecules, but also act as a compatibilizer between asphalt and SBS to promote the formation of a more stable network structure in PRM/SBS composite modified asphalt. Based on the above mechanism, the addition of PRM significantly improves the thermal storage stability of SBS modified asphalt. PRM/SBS composite modified asphalt has higher resistance to high-temperature permanent deformation than SBS modified asphalt with the same modifier content, and it presents a good low-temperature performance.