Abstract Water-resistant composite plastics were prepared from soy protein isolate (SPI) or soy dreg (SD), poly( ɛ -caprolactone) (PCL) and toluene-2,4-diisocyanate (TDI) as the compatibilizer by blending and one-step reactive extrusion (REX) followed by compression-molding. The structure and properties of the materials (SPI series and SD series) were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, dynamic mechanical thermal analysis and tensile testing. The results indicated that SPI and SD series exhibited high water resistance and good tensile strength (14.8 MPa for SPI35 and 16.3 MPa for SD35). Moreover, the SD series sheets containing cellulose possessed higher tensile strength than those of SPI series when SD content was 30–35%, whereas the latter had a better biodegradability and water resistance. By burying the two series of sheets in soil and culturing them in a mineral salt medium containing microorganisms, both of them were almost completely degraded. This paper provides a convenient way to prepare new soy protein plastics with good biodegradability and water resistivity.