A soybean protein isolate (SPI) main chain grafted with methyl acrylate (MA) and methyl methacrylate (MMA) as a new thermoplastic copolymer (T-SPI) was prepared. The properties of the material were evaluated with DSC, FT-IR, and rotary rheometry. The results indicated that the monomer was grafted on SPI successfully. The T-SPI had a glass transition at about 66°C. T-SPI was a typical viscoelastic material, and its elastic ratio was 65.27%, the shear viscosity was very sensitive to temperature and the flowability of T-SPI was regulated by temperature.