Abstract: A novel water-soluble sizing agent was designed for surface modification of aramid fiber. The synthesized sizing agent is environment-friendly, and the original modification process could be maintained with the new sizing agent. The sizing agent can repair defects and damages on the surface of aramid fiber to a certain extent, which could increase the mechanical properties of fiber reinforced composites. The sizing agent can self-assemble into a monomolecular layer on the aramid fiber surface, which can participate in the curing of epoxy to increase the interfacial shear strength (IFSS) of composites. The interfacial shear strength has a 23.81% increase after sizing. This method could improve the shear strength of the interface between aramid fiber and epoxy resin without damaging aramid fiber, and repair the defects on the fiber surface.