We successfully prepared a series of transparent blend films from castor oil-based polyurethane (PU) and p-phenylene diamine soy protein (PDSP). The miscibility, morphology, and properties of the blend films were investigated with Fourier transform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, scanning electron microscopy, moisture adsorption, thermal degradation, and tensile testing. The results revealed that the PDSP exhibited certain miscibility with PU varied its content from 10 to 80 wt % and also showed the strong hydrogen-bond and chemical cross-linking interactions lied between PU and PDSP. With an increase in the PU content, the elongation at break, thermal stability, and water resistance were improved whereas the tensile strength and Young's modulus decreased. It is worth noting that modified soy protein could be blended with hydrophobic polyurethane to obtain the blend films having good mechanical properties and optical transmittance.