Polyurethane (PU) has received extensive attention in structural engineering due to its excellent mechanical properties and damping properties. In this research, graphene-modified PU-epoxy composites are prepared for the application in structural engineering. The mechanical properties and damping properties of the composites with different contents of graphene are investigated. The damping properties of the composites were also investigated by performing dynamic mechanical analysis (DMA). The results showed that with the increase of graphene content, the tear strength of the composites decreased from 52 to 39 MPa. Due to the enhancement of appropriate amount of graphene, the tensile strength and elongation at break increase to the maximum value, being 16 MPa and 675%, respectively. And DMA tests showed that the damping properties reach the optimal values when the graphene content is 0.2%. The large damping temperature range (ΔT0.5) is 33 °C, from -2.9 to 30.1 °C. The peak of loss factor (ηmax) is 0.92 and the integral area (TA) is 36 °C, much higher than those of the pure PU-epoxy composites. In addition, scanning electron microscopy (SEM) results show that agglomeration appears with higher graphene content.