The waterborne polyurethane (WPU) modified by epoxidized soybean oil (ESO) and styrene (St) (WPU-ESO-St) was synthesized by free-radical emulsion polymerization using styrene (St) as a monomer, and azodiisobutyronitrile (AIBN) as initiator. The research on the mechanical properties of the film showed that with the increasing of the [NCO]=[OH] ratio, the contents of ESO, DAAM and the St, the tensile strength increased, and elongation at break decreased. The surface morphology of the film was characterized by atomic force microscopy (AFM). The glass transition temperature (Tg) of the film was characterized by differential scanning calorimetry (DSC) analysis.