Abstract: The corrosion inhibition of enamine ethyl imidazoline (IM) to 20G carbon steel, a key material of closed cooling water (CCW) system, was studied by immersion corrosion test, electrochemical test and surface analysis under the simulated operating conditions of CCW system in nuclear power plants, and its inhibition mechanism was analyzed. The thermal stability analysis of IM in the CCW system environment was investigated by thermogravimetric analysis. When the mass fraction of IM was 30 mg/kg, the inhibition efficiency of IM to 20G carbon steel could reach more than 99%. Adding 60 mg/kg IM into demineralized water had the same corrosion inhibition effect as adding 75 mg/kg sodium phosphate (corrosion inhibition efficiency more than 99.9%). With the increase of IM content, the free corrosion potential of 20G carbon steel moved forward and the corrosion current density decreased. The IM molecules began to decompose at about 240 ℃, and the decomposition was basically completed at 500 ℃. IM is an anodic corrosion inhibitor, which alleviates corrosion by adsorbing and forming a protective film on the surface of 20G carbon steel.