Polymer modification is an important way to improve the high temperature performance of asphalt. In this work, PU-SBS composite modified asphalt was prepared by PU and SBS， and the effect of PU-SBS on high temperature performance of asphalt and modification mechanism were studied. The results show that PU-SBS composite modification can significantly reduce the penetration of asphalt and improve the softening point. PU-SBS modified asphalt can effectively improve the viscosity and high temperature rheological properties of asphalt, and the modification effect is obviously better than that of single polymer modification. PU-SBS modified asphalt has good permanent deformation resistance and elastic recovery at high temperature. The rutting resistance factor G*/sin δ at 64 °C reaches 18.35 kPa, which is 2.6 times and 1.6 times of SBS modified asphalt and PU modified asphalt under the same conditions, respectively. In addition, PU-SBS composite modification combines physical and chemical modification to improve the high temperature performance of asphalt: physical adsorption swelling of SBS in asphalt and chemical reaction of PU with asphalt components.