In order to seek wider fields of application of carboxymethyl starch (CMS), the inhibition of CMS on mild steel in hydrochloric acid (HCl) solution has been investigated by polarisation curves and electrochemical impedance spectroscopy. The results show that CMS has a good tendency as a corrosion inhibitor in 5% hydrochloric acid solution at room temperature, 40°C and 60°C. When CMS with a concentration of up to 2 g/l was added, the corrosion rate decreased to 11·19 mm/year and the inhibition efficiency was 80·61%. The corrosion rate increased with the increase in temperature of the medium. CMS is found to be a kind of cathode corrosion inhibitor, and the inhibition efficiency increases with increasing concentration of CMS, and the inhibition efficiency is best at room temperature. Passivation will be accelerated on the surface of the electrode when mild steel electrode is in 5% hydrochloric acid solution at 40°C, which leads to good resistance to corrosion.