In order to explore high-performance grouting materials suitable for small-size cavity behind tunnel lining， 11 groups of polyurethane grouting materials were prepared， and their mechanical properties were studied. An improved constitutive equation was proposed， and the grey correlation was used for comprehensive evaluation. The results show that 1.16 %-1.94 % of catalyst and 3.80%-5.60% of chain extender can effectively improve the mechanical properties of polyurethane. The introduction of high molecular weight polyols reduces the compressive strength and tensile strength of polyurethane， but significantly improves its bonding strength. The constitutive model based on the phenomenological model can accurately characterize the compressive deformation of polyurethane. Based on the grey correlation， the optimal mix proportion of polyols， isocyanates and a relative molecular weight of 8 000 and 500 is 25：2：23， with a catalyst content of 1.16% and a chain extender content of 1.94%. Under this optimal mix ratio， the polyurethane has the best comprehensive performance.