AbstractA method of using modified epoxy resin (PEG‐E51) and graphene oxide (GO) as sizing agent in different pH has been proposed and applied to improve the interfacial interactions and bonding of carbon fiber (CF) reinforced epoxy resin‐based (EP) composites. The PEG‐E51 and GO complex sizing in different pH is prepared and then coated on CF surfaces homogeneously. The sizing layer forms on the fiber surfaces, and multiple GO sheets are introduced successfully surrounding the CFs. The surface morphologies of CFs change distinctly with different pH. The interfacial shear strength increases from 51.36 MPa for bare fiber‐reinforced EP composites to 66.62 MPa for composites reinforced by CFs coated with GO only. However, a significant improvement is achieved when the pH is adjusted to 10, making the interfacial shear strength grow up to 77.23 MPa. Furthermore, scanning electron microscopy results and interlaminar shear strength test are in agreement with each other, suggesting better interface bonding of composites by regulating pH of PEG‐E51 and GO complex sizing. Besides, the interfacial interaction mechanism in CF reinforced composites is also explored, that is, the positive effects of roughness and specific surface area in complex interfacial layers lead to the improvement of interfacial properties.