Walnut green husk inhibitor (WGHI) was extracted and prepared by reflux extraction with the solvent of ethanol water solution. The inhibition action of WGHI on the corrosion of cold rolled steel in 0.5 mol/L HCl was studied by weight loss, potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), ultraviolet and visible spectrophotometer (UV), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) methods. The results show that the optimum volume fraction of ethanol solution is 40%, and the corresponding maximum production rate and inhibition efficiency are 14.2% and 90.3%, respectively. WGHI can efficiently inhibit the corrosion of cold rolled steel in 0.5 mol/L HCl. Inhibition performance increases with the concentration of WGHI, but decreases with the increase of temperature. Through both physisorption and chemisorption mechanism, WGHI can adsorb on steel surface spontaneously, in which the heat is released and accompanied by a decrease in chaotic degree, and its adsorption behavior obeys Langmuir adsorption isotherm. The addition of WGHI drastically inhibits both anodic and cathodic reactions for the corrosion of steel, and can be arranged as a mixed-type inhibitor. Nyquist appears an individual depressed capacitive loop, and the impedance increases with the increase of WGHI concentration. Inspection of the micrographs of corroded steel surface reveals that WGHI can efficiently retard the corrosion and decrease the surface roughness of steel surface in HCl solution.