Abstract: The interfacial properties of high-modulus carbon fiber (HMCF) composites have always been the key to their applications, and surface modification of HMCF is one of the important ways to improve the interfacial properties. Graphene nanosheets (GNP) were introduced into the surface of HMCF by electrophoretic deposition, and the surface morphology and surface chemical state of HMCF before and after modification were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), and the interlaminar shear strength (ILSS) and moisture absorption rate of HMCF reinforced epoxy resin composites before and after modification were tested. The results show that the surface electrophoretic deposition of GNP by HMCF can not only improve the interfacial properties of the reinforced epoxy resin composites, but also reduce the moisture absorption rate of the composites. When the GNP concentration was 0.5 mg/mL and the electrophoretic deposition voltage was 10 V, the interlaminar shear performance of the composites increased by 8.8% to 75.6 MPa, and the moisture absorption rate was reduced by 9.5% compared with the unmodified sample after 30 days of storage at 90°C and 80 RH%, and the interlayer shear performance remained at 67.8 MPa after 60 days of storage. Both interfacial and hygrothermal properties of the modified HMCF comosites have been improved.