Abstract: The prepolymer of hydrophilic polyurethane is synthesized by diisocyanate and polyether polyol, and then the polyurethane prepolymer blends with sodium silicate solution to prepare nanosilica-reinforced hydrophilic polyurethane composites (HPU/nano-SiO₂) via in-situ reactions. The FTIR-ATR and X-ray diffraction (XRD) are employed to confirm the existence of the nano-SiO₂ and its chemical bond to HPU chain, and the size and dispersion of nano-SiO₂ in HPU matrix are investigated by transmission electron microscopy (TEM). The gel time of the mixture consisting of HPU prepolymer and sodium silicate solutions and the mechanical properties of HPU/nano-SiO₂ composites are also tested. The results show that the particles of nano-SiO₂ disperse homogeneously in the HPU matrix, and the nano-SiO₂ combines with HPU chains by chemical bonds to form HPU/nano-SiO₂ composites. What’s more, the gel time of HPU and sodium silicate solution blend increases compared to that of the blend of HPU and water, and the compression strength of HPU/nano-SiO₂ is improved effectively than that of pristine HPU, so the HPU prepolymer can react with sodium silicate solutions to produce the nanosilica reinforced hydrophilic polyurethane grout, which has high ability and efficiency to form adequate plugs for underground water though the plugging project of coal mine.