Abstract: A acetyl-terminated waterborne hyperbranched polyurethane(AWHBPU) was synthesized by acetic anhydride and waterborne hyperbranched polyurethane(WHBPU). The viscosity, particle size distribution and surface tension of waterborne polyurethane emulsion modified by waterborne hyperbranched polyurethane were studied. Results show that the viscosity of the modified emulsion can be reduced by AWHBPU blending, while non-Newtonian index of the emulsion modified by AWHBPU increases. The test results of particle size distribution and surface tension show that the main cause of the viscosity declining is the particle size increasing, which is induced by AWHBPU accessing to the inside of waterborne polyurethane micelle.