Abstract: A phosphated polyester polyol with a 1500 number average relative molecular mass was synthesized by 2-phosphono-1,2,4-butanetricarboxylic acid and 2,2-dimethyl-1,3-propane diol. Then the polyester polyol was reacted with isophorone diisocyanate to obtain the prepolymer. The prepolymer was extended by 1,4-butanediol and terminated by coupling agent KH550. At last nanosilica was added into the waterborne polyurethane(WPU) to obtain WPU/nanosilica composite. Fourier transform-infrared spectroscopy(FT-IR), thermal gravimetric analysis(TGA) and transmission electron microscopy(TEM) were used to study the structure and characteristics of the composite. In the TEM image of PWPU/SiO₂, plenty of composite particles(~110 nm diameter) were observed. Combined analysis with the FI-IR, it was concluded that the nanosilica was linked with WPU by chemical bond. TGA, char yield and extension test showed that the fire retardancy property, thermostability and tensile strength of the composite increase significantly with tnanosilica modification. These results indicate that there is a good application prospect of the composite in the future.