As environmental concerns caused by waste polyethylene terephthalate (PET) become increasingly severe, utilizing it as a resource for asphalt modification has emerged as an important direction in road engineering. However, existing studies have predominantly focused on PET-modified SBS or rubber asphalt individually, leaving the effects of PET-derived additives on SBS/CR composite modified systems inadequately understood. In this study, a PET chemical depolymerization product (PET EA) was prepared via aminolysis and incorporated into both low content (3% SBS + 5% CR) and high content (7% SBS + 10% CR) composite modified asphalts at various dosages. The influence of PET EA content on asphalt properties was systematically investigated, and fluorescence microscopy was employed to elucidate the modification mechanism. The results show that PET EA increases the softening point and viscosity of the SBS/CR modified asphalt systems, while adversely affecting ductility; these effects are more pronounced in the high content system. Specifically, at 7% PET EA, the softening point increases by 4.8?°C, the viscosity at 135?°C rises by approximately 185%, and the maximum tensile stress during ductility testing increases by 85%, whereas ductility decreases from 19.7?cm to 6.4?cm. Furthermore, PET EA acts as a stabilizer in the composite system, inhibiting sedimentation and phase separation of the modifiers during high temperature storage, thereby improving storage stability. The amide groups present in PET EA promote the migration of light components from the asphalt into the SBS phase, facilitating further swelling of SBS, which in turn affects macroscopic properties such as the three conventional indices and viscosity. Overall, PET EA can serve as an effective additive for SBS/CR composite modified asphalt, offering both performance enhancement and stabilization. The findings provide a theoretical reference for the preparation of PET SBS CR composite modified asphalt.