Cationic cassava starch graft copolymer (CCSGC) was prepared by reacting cassava starch (CS) with acryl amide (AA) and dimethyl diallyl ammonium chloride (DMDAAC). The corrosion inhibition of CCSGC on the corrosion of cold rolled steel in HCl was studied by weight loss, open circuit potential (OCP)-time curves, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) methods. The results show that CCSGC exhibits outstanding inhibition effect for cold rolled steel in 1.0 mol/L HCl, and the inhibition efficiency values are higher than 92% at 20 ~ 50 oC even when the additive CCSGC concentration is as low as 50 mg/L. The inhibitive performance of CCSGC is better than the reactants of AA, CS, DMDAAC or AA/CS/DMDAAC mixture. The adsorption of CCSGC on steel surface obeys Langmuir adsorption isotherm, and it is an exothermic process that involves mixed adsorption accompanied by an increase of chaotic degree. CSGC is a mixed-type inhibitor that retards both anodic and cathodic reactions simultaneously, and the electrochemical mechanism is caused by "geometric blocking effect". EIS exhibits the depressed capacitive loop, and the charge transfer resistance increases with the concentration of inhibitor, while double layer capacitance decreases. SEM reveals that the corrosion of steel is prominently retarded after adding CCSGC to the media.