Hydrophobic nano-SiO2 particles were prepared from reacting coupling agent KH-550 and nano SiO2 particles in the ultrasound, and dispersed in polyurethane solution modified by castor oil (CO). The performances of polyurethane were characterized by Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). The FT-IR analysis indicates the formation of chemical bond between PU and nano-SiO2, and formed the Si-O-Si particle structure. As m(CO)：m(N210)=1:9 the tensile strength o f the films increased from 6.8 MPa to 16.2 MPa. The water absorption also increased two times. As the mass fraction of nano-SiO2 increased from 0 to 2%, the initial weight loss temperature of the PU films increased from 255 e to 272 e. The films performance is better, greatly improve the mechanical properties, water resistance and heat resistance.