Walnut green husk extract (WGHE) was obtained from the raw materials of forestry and agricultural residue of walnut green husk by circulation reflux method with ethanol water solution as extractive solvent. The synergistic inhibition performance and mechanism of WGHE and potassium iodide (KI) on the corrosion of a cold rolled steel (CRS) in 1.0 mol/L citric acid (H₃C₆H₅O₇) solution were assessed by means of immersion test, potentiodynamic polarization curve measurement, and electrochemical impedance spectroscopy (EIS), as well as scanning electron microscope (SEM) and atomic force microscope (AFM). The results showed that either individual WGHE or KI was a moderate inhibitor, and the maximum inhibition efficiency was 67.4% for WGHE and 69.3% for KI respectively. When WGHE was mixed with KI, the complex could promote the inhibitive action, and the synergism parameter was higher than 1 for the solution at 20 ℃. There was a synergism between WGHE and KI, and the maximum inhibition efficiency for the mixture of 200 mg/L WGHE with 100 mg/L KI was 85.3%. No matter when WGHE was mixed with KI, the adsorption of inhibitor molecules on CRS surface obeys Langmuir isotherm. The incorporation of WGHE with KI further increases the adsorption equilibrium constant (K) up to 0.0382 L/mg，while the standard Gibbs adsorption free energy (ΔG⁰) shifts more negatively to -28.9 kJ/mol. Potentiodynamic curves indicate that individual WGHE or KI mainly retards the cathodic reaction, while the mixture of WGHE/KI acts as a mixed-type inhibitor. SEM and AFM microphotographs confirm that the synergistic of WGHE and KI can more efficiently alleviate the citric acid induced corrosion of the cold rolled steel.