In this study, we aim to improve the surface property and bulk tensile property of M40J graphite fiber by gamma-ray co-irradiation treatment of the fiber in epichlorohydrin/acetone mixed solution. The effect of the irradiation treatment was analyzed by X-ray photoelectron spectroscopy, atomic force microscopy, dynamic contact angle test, tensile test, short-beam shear test, scanning electron microscopy, and X-ray diffraction, respectively. The results indicated that the reactive activity, roughness, and interfacial adhesion property of the fiber were improved, and without destroying the bulk structure after the surface irradiation grafting treatment. Mechanical testing results indicated that the tensile strength was decreased slightly; however the tensile module was improved. The gamma-ray co-irradiation grafting was an effective method for modifying the physicochemical properties of M40J graphite fiber and improving the interfacial adhesion of its composites.