AbstractThis study examined a method to improve the water stability of macroporous polyurethane mixtures. Poor water stability of these mixtures has become an obstacle to their popularization and practical application. The water stability of asphalt mixtures can be significantly improved by adding a silane coupling agent (SCA), but the application of this method to polyurethane mixtures has not been researched. Therefore, this study investigated the addition of an SCA to polyurethane to enhance the interfacial adhesion between the polyurethane and the aggregate, thereby improving the water stability of the target macroporous polyurethane mixture. To achieve this aim, two SCAs, KH550 and KH560, were selected, and the change in the bond strength of polyurethane with 0%–4% SCA content was analyzed. The results were then compared and verified in accordance with surface free energy theory. The influence of the SCA on the water stability of the macroporous polyurethane mixture was verified by performing an immersion Marshall test, freeze–thaw splitting test, and immersion Cantabro loss test. Additionally, the effects of SCA on the high-temperature performance and skid resistance of the macroporous polyurethane mixture were analyzed by performing rutting and skid-resistance tests. The results revealed that the bond strength of polyurethane initially increased, and then decreased, with increasing SCA content. The effect of KH550 on bond strength was significantly greater than that of KH560, because KH560 does not react with polyurethane. Thus, according to the bond strength results, the recommended SCA was KH550, and the optimal SCA content was 2%. The immersion Marshall and freeze–thaw splitting tests were determined to be more suitable for evaluating the water stability of macroporous polyurethane mixtures than the immersion Cantabro loss test. Compared with the ordinary macroporous polyurethane mixture, the tensile strength ratio and residual stability of the modified mixture with 2% KH550 were 35% and 12.4% higher, respectively, indicating that the addition of KH550 significantly improved the water stability of the macroporous polyurethane mixture. Additionally, the tensile strength ratio results were found to be consistent with the surface free energy theory. Furthermore, the addition of KH550 slightly enhanced the skid resistance and high-temperature performance of the macroporous polyurethane mixture.