Anionic waterborne epoxy resin (AAEK) was synthesized by using epoxy resin (EP) and adipic acid as reagents and KOH as neutralizing agent. The AAEK possesses hydrophilicity, which can be employed as sizing agent for carbon fiber (CF), and then the CF/EP composites were obtained by mixing the treated CFs and EP. In addition, CF treated by AAEK（CF-AAEK）and CF-AAEK composite were investigated by infrared spectroscopy (IR), scanning electron microscope (SEM), atomic force microscopy (AFM), adsorption testing and universal testing machine. The results indicate that the optimum concentration and adsorption amount are 1.0% and 3.0mg/g respectively; the dispersion of short CFs in EP is dramatically improved after AAEK treatment; AAEK adsorption on the CF surface is an endothermic process between monolayer and multilayer, which consists with the adsorption model of Freundlich; the monofilament breaking strength of CF-AAEK increases slightly compared with the untreated CF. the flexural strength and interlaminar shear strength (ILSS) of CF-AAEK composite are proved to increase by 168% and 139% compared with untreated CF.