The application of high-viscosity modifiers in asphalt modification is receiving increasing attention for facilitating the development of drainage pavements. However, compatibility issues and phase separation between modifiers and asphalt remain major challenges that restrict large-scale field applications, with the deterioration of rheological properties being a direct manifestation of insufficient compatibility. In this study, a high-viscosity asphalt modifier based on a styrene–butadiene–styrene block copolymer/terpene resin blend (SBS/terpene resin blend [STRB]) was prepared via direct injection using a polymer blending method. The chemical characteristics of STRB and its compatibility mechanism with asphalt were investigated at multiple scales through solubility parameter analysis, Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), surface energy analysis, and fluorescence microscopy, and compared with the two conventional modifiers, TAFPACK-Super (TPS) and Tianyi (TY). The modification effects were evaluated through comprehensive performance tests, including dynamic shear rheometer (DSR) testing, on STRB-modified high-viscosity asphalt (HVA-B), TPS-modified HVA (HVA-S), and TY-modified HVA (HVA-Y). The results demonstrate that, compared with the two conventional modifiers, STRB contains more oxygen-containing functional groups, exhibits higher polarity, and possesses superior compatibility with asphalt. This enhanced compatibility enables STRB to disperse more uniformly in asphalt with a smaller average particle size, thereby significantly improving its rheological properties. Consequently, the mechanical properties and high-temperature rutting resistance of HVA-B are markedly enhanced compared to HVA-S and HVA-Y. This study provides theoretical insights and technical references for the development of high-compatibility asphalt modifiers and has significant implications for advancing the engineering application of drainage pavements.