<p>The aging mechanisms of styrene–butadiene–styrene block copolymers (SBS)-modified bitumen under varying aging environments, particularly in oxygen-deficient or anaerobic conditions, remain insufficiently clarified. In this study, the effects of thermal-oxidative and vacuum aging on SBS-modified bitumen are investigated systematically by analyzing changes in chemical composition, functional groups, microstructure, thermal performance and dynamic rheological properties. Results reveal that during the initial aging stage (&lt; 40&#xa0;h), the base bitumen forms a protective barrier, effectively delaying SBS degradation. Thermal-oxidative aging increases the asphaltene content accompanied by a reduction in the quantity of bee-shaped microstructures but an increase in their average size, yielding a enhanced elastic binder (decreased phase angle, <i>δ</i>). In later stages, rapid SBS depolymerization via oxidative chain scission occurs (near-complete decomposition after 100&#xa0;h), leading to the disruption of the SBS network and reducing elasticity (increased <i>δ</i>). This aging-driven transition shifts rheological dominance from SBS network reinforcement to bitumen matrix hardening. In contrast, vacuum aging preserves partial SBS structures, which critically sustain the modified bitumen’s elasticity. The retained SBS restricts asphaltene molecular migration, increasing the number of bee-like structures while reducing their individual sizes, thereby delaying bitumen hardening and significantly enhancing the rutting resistance parameter (<i>G</i>^*/sin<i>δ</i>). This work advances the understanding of SBS-modified bitumen aging pathways under diverse service conditions.</p>