In order to enhance the mechanical properties of polyethylene glycol (PEG) polyurethane elastomer, the modified hyperbranched polyester (HBP) was introduced. The chemical structures of HBP/PEG polyurethane films were analyzed by attenuated total reflection-Fourier transfoum infrared spectrum (ATR-FTIR). The results indicate that with the modified hyperbranched polyester added, the tensile strength and elongation at break of PEG polyurethane elastomer are improved obviously. When the content of hyperbranched polyester of the third generation is 0.4%, the resultant elastomer has the best tensile strength, which increases 2.53 times. When the content of hyperbranched polyester of the first generation rises up to 1.6%, the resultant elastomer has the best elongation at break, which increases 1.43 times. The degree of the total hydrogen bonding and the degree of microphase separation are increased, which enhance the mechanical properties of PEG polyurethane elastomer.