In this work, experimental and theoretical methods were used to investigate the effect of hyperbranched epoxy resin (HPE) grafting on the interfacial and mechanical properties of carbon fiber (CF)/epoxy composites. Experimentally, the HPE was grafted onto the CF surface by chemical grafting method to prepare the CF-HPE reinforcement. Results show that compared with the unmodified CF, the surface roughness and wettability of CF-HPE reinforcement increase significantly. The CF/epoxy composites were prepared by compression molding method. Compared with the unmodified CF reinforced epoxy composites, the interfacial shear strength (IFSS), the interlaminar shear strength (ILSS) and the flexural strength of CF-HPE/epoxy composites are increased by 117.2%, 34.2% and 13.5%, respectively. Furthermore, the HPE grafting can also improve the thermal stability of CF/epoxy composites. The behind enhancement mechanisms can be attributed to the improvement of surface roughness and wettability of CF-HPE, thereby further enhancing the interfacial interaction and mechanical interlocking between CF-HPE and epoxy resin. In addition, the epoxy groups in HPE can react with the epoxy resin matrix and form stronger chemical bonding between the CF and epoxy. Theoretically, molecular dynamics (MD) method was used to reveal the internal mechanism of enhancing the interfacial properties of CF-HPE/epoxy composites from the molecular level. Results show that HPE grafting can improve the interfacial interaction energy between CF and epoxy, and decrease the mean-square displacement (MSD) value and the free volume fraction of CF/epoxy composites. This work provides valuable insights to the material design of epoxy composites containing reinforcements grafted with HPE.