<p>A polyurethane semi-prepolymer was introduced as a new modifier for asphalt. The performance of the polyurethane semi-prepolymer-modified asphalt were compared with SBS-modified asphalt. The results of infrared spectroscopy and fluorescence microscopy indicate that there is a chemical reaction between the polyurethane semi-prepolymer and the asphalt, while the SBS and the asphalt are physically cross-linked. The dynamic shear rheometer (DSR) tests indicate that the polyurethane semi-prepolymer-modified asphalt exhibits resistance to deformation at high temperatures, remarkable elastic recovery, minimal stress sensitivity, and higher viscosity. However, its low-temperature performance is inferior to that of SBS-modified asphalt. Based on the asphalt mixture rutting test, the study confirms the excellent rutting resistance of polyurethane semi-prepolymer-modified asphalt. Moreover, the three-point beam bending test and semi-circular bending test (SCB) indicate that the low-to-medium temperature cracking resistance of the polyurethane semi-prepolymer-modified asphalt mixture is inferior to that of SBS-modified asphalt. Therefore, polyurethane semi-prepolymer-modified asphalt exhibits significant application potential in high-temperature regions and on heavy-duty roadways.</p>