The invention relates to a preparing method of bifunctionality epoxy resin emulsifier and emulsion. The preparing method comprises the specific steps that 1, trimellitic anhydride (TMA) and a catalyst (boron trifluoride diethyl etherate) are added into polyethylene glycol (PEG4000), and the temperature is raised for reaction; 2, a proper amount of bisphenol A epoxy resin E-44 is added into the reaction liquid, and the temperature continues to be raised for the reaction, so that multi-block epoxy resin emulsifier (PEGDET for short) is obtained; 3, a proper amount of epoxy resin and the emulsifier are mixed, deionized water is slowly added dropwise at a certain temperature, high-speed stirring is carried out until viscosity of a system is reduced abruptly, at the moment, the system is subjected to phase inversion, and water continues to be added, so that the water-borne epoxy resin emulsion is obtained. The water-borne epoxy resin emulsion prepared through the method has the advantages of being small in average grain diameter, low in dispersion, excellent in centrifuge, dilution and storage stability performance and the like, and a new thought is provided for preparing high-performance water-borne epoxy emulsion.