To improve the performance of the epoxy resin, isophorone diisocyanate (IPDI)-modified graphene oxide (GO) nanosheets were used, and the modified graphene oxide (iGO) was grafted with epoxy resin using polyurethane (PU) chain segments as a bridge to prepare epoxy composites. The iGO was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction analysis (XRD), and transmission electron microscopy (TEM), and the results showed that IPDI increased the layer spacing of iGO from 0.79 nm to 1.06 nm by grafting with hydroxyl and carboxyl groups on the surface of GO nanosheets, which improved the degree of exfoliation of iGO. Tensile testing, dynamic thermomechanical analysis (DMA), and scanning electron microscopy (SEM) were used to figure out the thermomechanical properties of the modified epoxy composites. The results show that the tensile strength and elongation at break of the as-prepared 0.5% iGO-PU/EP composites are 0.2 and 6.3 times higher than those of pure EP, respectively, and that the glass transition temperature was raised by about 2°C, with a significant increase in crosslinking.