With the gradual shift of high-grade highways in China from construction to maintenance, higher performance requirements have been placed on asphalt binders. High-content SBS modified asphalt has become an inevitable choice for both new pavement construction and maintenance. However, conventional high-content SBS modified asphalt suffers from high energy consumption, excessive carbon emissions, and poor construction workability. In this study, a self-developed warm-mixed additive was introduced into high-content SBS modified asphalt, and the process was optimized to obtain warm-mixed SBS modified asphalt. The effects of the additive on the asphalt performance and the warm-mix efficiency were evaluated in terms of viscosity-temperature characteristics, rheological properties, and thermal properties, while the viscosity-reduction mechanism was further revealed through microstructural analysis. The results show that when the mixing ratio of additive A to B is 2% to 1%, the warm-mixed SBS modified asphalt exhibits optimal performance. The softening point increases by 1.8 ℃; the ductility at 5 ℃ improves by 5.7 cm, and the rotational viscosity at 135 ℃ decreases by 0.9 Pa·s, thereby significantly enhancing construction workability. Rheological tests demonstrate that both high- and low-temperature performance meet the PG76-22 grade requirements. Microstructural observations confirm that no new chemical substances are generated during the viscosity-reduction process; instead, the additive functions as a lubricant in the molten state to reduce viscosity through physical action and serves as a skeleton in the solid state to reinforce the binder and improve its rheological properties.