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Cassava starch ternary graft copolymer of cassava starch-sodium allylsulfonate-acryl amide graft copolymer (CS-SAS-AAGC) was synthesized through cassava starch (CS) chemically modified by grafting two monomers of sodium allylsulfonate (SAS) and acryl amide (AA) simultaneously. The inhibition performance of CS-SAS-AAGC on cold rolled steel (CRS) in 1.0 M HCl solution was experimentally studied by weight loss, electrochemical techniques and surface analysis, and its adsorption was theoretically investigated by both quantum chemical calculation and molecular dynamic (MD) simulation. The results indicate that CS-SAS-AAGC shows an optimum inhibition efficiency as high as 97.2% at 50 mg L−1, and its inhibitive ability exhibits more stronger than that of CS, SAS or AA. CS-SAS-AAGC behaves as a mixed-type inhibitor through geometric blocking effect. SEM and AFM images clearly reveal that the corrosion of CRS surface is efficiently retarded by CS-SAS-AAGC. Contact angle result suggests that the inhibited CRS surface is of hydrophobic nature. The adsorption active sites of CS-SAS-AAGC are mainly the grafted monomers of SAS and AA. Keywords: Corrosion inhibitor, Hydrochloric acid, Steel, Ternary graft copolymer, Adsorption