Abstract: Juglans regia green husk inhibitor was prepared from forestry and agricultural residue of walnut green husk by using the extraction solvent of ethanol water solution. The synergistic inhibition effect of walnut green husk inhibitor and nonionic surfactant of polyethylene glycol mono-(p) octyl phenyl ether (OP) on the corrosion of cold rolled steel (CRS) in dichloroacetic acid (Cl₂HCCOOH) solution was studied by weight loss and potentiodynamic polarization curves, electrochemical impedance spectra and scanning electron microscope methods. Effects of composite inhibitors concentrations, temperature, immersion time and dichloroacetic acid concentration on synergistic inhibition performance were discussed in detail. The results show that J. regia green husk inhibitor exhibits moderate inhibition with the maximum inhibition efficiency (η_w) of 200 mg/L is 65%, and OP acts as a poor inhibitor with the maximum inhibition efficiency (η_w) of only 40%. However, incorporation of J. regia green husk inhibitor with OP can obtain better inhibitive performance, and the maximum inhibition efficiency (η_w) is higher than 93%. There is a strong synergistic inhibition effect for J. regia green husk inhibitor and OP. The synergistic performance increases with the increase of J. regia green husk inhibitor concentration, immersion time and acid concentration in general, while decreases with an increase of temperature in general. The adsorption of J. regia green husk inhibitor, OP or J. regia green husk inhibitor/OP on steel/Cl₂HCCOOH interface follows Langmuir adsorption isotherm, and the adsorption equilibrium constant increases for the composite inhibitors. The standard adsorption Gibbs free energy value for J. regia green husk inhibitor/OP is −28.2 kJ/mol. Either J. regia green husk inhibitor or OP is a cathodic inhibitor, while the mixture of walnut green husk inhibitor/OP acts as a mixed-type inhibitor that more efficiently retards both cathodic and anodic reactions. Nyquist spectrum exhibits a depressed capacitive loop, and the presence of J. regia green husk inhibitor/OP significantly increases the charge transfer resistance. Scanning electron microscope clearly confirms J. regia green husk inhibitor/OP can efficiently alleviate the corrosion degree of steel surface.