Due to the unclear constitutive relationship between the structure and performance of styrene-butadiene-styrene block copolymer(SBS) modified asphalt, the current way to improve the performance of SBS modified asphalt is still to simply increase its SBS content. However, early pavement diseases are still frequent. To explore the effect of swelling degree of SBS on the rheological properties of modified asphalt and its internal mechanism without increasing SBS content. The microstructure of SBS modified asphalt was observed by fluorescence microscope. The conventional properties and rheological properties of SBS modified asphalt were analyzed by dynamic shear rheometer. The internal mechanism of the influence of SBS swelling degree on the performance of SBS modified asphalt was revealed by molecular dynamics. The results show that the fully swollen star-line blended SBS modified asphalt has a higher swelling area, and has obvious performance advantages in terms of conventional performance, rheological properties and anti-aging properties. Molecular simulation shows that the complete swelling of SBS makes the radial distribution function peak of SBS modified asphalt higher, which improves the interaction between SBS molecules and light components in SBS modified asphalt. On the basis of maintaining the original stable asphalt colloid structure, SBS styrene ends are interconnected to form π-π conjugate, which improves the toughness of SBS network.