Abstract: In order to obtain waterborne epoxy resin with a high surface activity, the epoxy resin was modified by phosphorus pentoxide as a phosphorylation reagent, and the phosphorylation process was optimized. The effect of reaction conditions on the epoxy value, monoester content and diester content of resultant product was investigated with respect to the main influence factors such as reaction feed ratio, reaction time and temperature. The optimum reaction conditions were determined as an E20/P2O5 molar ratio of 1.2/1, a reaction temperature of 85 ℃ and reaction time of 3 h. The synthesized product achieved an epoxy value of 0.057 mol/100 g, a monoester content of 0.44 wt % and a diester content of 0.56 wt % under these conditions. The chemical structure of modified epoxy resin was characterized by Fourier-transform infrared spectroscopy and NMR spectrometer, and their thermal stability, viscosity-temperature, viscosity-concentration, particle size and surface activity were also investigated. The results indicated that there was partial breakage of epoxy bonds occurring in the molecular structure of the modified epoxy resin, followed by an appearance of monoester and diester functional groups of phosphoric acid. These results suggested that this new modified epoxy resin was synthesized successfully, and it exhibited a good thermal stability and a small average particle size in emulsion is smaller. When the mass fraction of emulsion was lower than 10 wt %, the epoxy resin showed a high surface activity and its viscosity was almost not influenced by the mass fraction. Moreover, the emulsion presented a minimum surface tension at a mass fraction of 0.5 wt %.