Due to the low molecular weight of traditional linear waterborne polyurethane (WPU), there are a series of shortcomings in practical application, such as low mechanical properties, poor thermal resistance, and poor water resistance. In this paper, a high-crosslinking cationic waterborne polyurethane emulsion was prepared by using polytetramethylene ether glycol (PTMG) and isophorone diisocyanate (IPDI) as soft segments and hard segments, respectively, castor oil (CO) as a precrosslinking agent, and polyethyleneimine (PEI) as a macromolecular postcrosslinking agent. The effects of the addition amount of PEI on the properties of WPU emulsions and their films were studied. The properties of WPU emulsions were characterized by a viscometer, pH meter, high-speed centrifuge, and laser particle size analyzer. The viscosity of the WPU emulsion is 5–12 mPa·s, the pH is 2.69–3.03, the solid content is about 35%, and the particle size is 80–150 nm. The WPU films were prepared by a casting method. The structures, thermal properties, mechanical properties, and surface wettability of the WPU films were characterized by Fourier transform infrared spectroscopy (FTIR), crosslinking density measurement, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) and by an electronic universal testing machine and contact angle-measuring instrument. The results show that the crosslinking density, thermal stability, contact angle, and tensile strength of WPU films increased with the increase in PEI content, but the elongation at break increased first and then decreased.