Soy protein based materials are of great interest because of the merits of biocompatibility, biodegradability, renewability, etc. However, the poor mechanical properties and high water sensitivity limit their further application in many fields. In this paper, we tried to overcome these shortcomings through a slight chemical modification of the polypeptide chains of soy protein. P-31 NMR and solid state C-13 CP/MAS NMR spectroscopy confirmed that the diethoxy phosphoryl groups were successfully grafted onto soy protein chains with a molar grafting ratio of 0.15-1.18%, which almost did not change the nature of soy protein. The isoelectric point and rheological behavior of the modified soy protein sample varied with the grafting ratio, indicating that the tertiary structure of the protein was changed after phosphoryl modification. The FTIR spectra of the modified soy protein suggested that the increase of beta-sheet conformation from the slight chemical modification could be the reason for the change of the globular structure of soy protein. Finally, we obtained a robust soy protein film as expected, and we did not use any crosslinking agent and plasticizer that were almost unavoidable in the previous studies reported in the literature. The tensile strength and the elongation at break of our soy protein films were 35 +/- 5 MPa, 2.5 +/- 0.5% in the dry state, and 3.8 +/- 1.5 MPa, 125 +/- 5% in the wet state, respectively. We believe that the method we developed in this communication provides a practical approach to improve the mechanical properties and broaden the applications of natural soy protein based materials.