In order to improve the thermo-oxidative stability of three-dimensional and four-directional braided carbon fiber/epoxy composites, we introduced a gradient interphase reinforced by graphene nanoplatelets (GN) between the carbon fiber and the matrix, with a liquid phase deposition strategy. Both the interlaminar shear strength and the flexural strength of the composites were improved after thermo-oxidative aging at 140掳C for various durations (up to 1200h). The interfacial reinforcing mechanisms are explored by analyzing the structure of the interfacial phase, thermal conductivity, weight loss, surface topography, fiber/matrix interfacial morphology and thermomechanical properties of the composites. Results indicate that the GN-reinforced gradient interphase provides an effective shield against interface oxidation, assists in thermal stress transfer, and restricts the movement of the different phases of materials at the composite interface.