The work aims to study the synergistic inhibition effect of walnut green husk extract (WGHE) and sodium dodecyl sulfate (SDS) of anionic surfactant on the corrosion of cold rolled steel (CRS) in H3PO4 solution, so as to reveal the synergistic inhibitive action rule and discuss the synergistic inhibitive mechanism. WGHE was extracted and prepared by reflux extraction method from the agricultural waste of walnut green husk. The synergistic inhibitive action of WGHE and SDS, electrochemical mechanism, ultra-violet (UV) absorption curve and micro-graphs of steel surface were studied by weight loss, electrochemical technique, ultra-violet spectrum (UV), scanning electron microscope (SEM) and atomic force microscope (AFM) . WGHE exhibited better inhibitive performance at higher concentration for CRS in 1.0 mol/L H3PO4 solution, but poor inhibition at lower concentration. The maximum inhibition efficiency of individual SDS was below 60%. Incorporation of WGHE with SDS could further improve the inhibitive performance, and there was a strong synergistic inhibition effect for the WGHE concentration range of 10~40 mg/L. Inhibition efficiency increased with the increase of additive inhibitor dosage, while decreased with increase of temperature. The adsorption of WGHE, SDS or WGHE/SDS on CRS surface followed Langmuir adsorption isotherm. Potentiodynamic curves indicated that both WGHE and SDS could be arranged as the cathodic inhibitors, while the mixture of WGHE/SDS acted as a mixed-type inhibitor. Nyquist spectra showed a depressed capacitive loop at high frequencies, while a small inductive section at low frequencies. The charge transfer resistance value increased prominently after the mixture of WGHE/SDS was added. An interaction occurred between WGHE and SDS in synergetic system. SEM and AFM microphotographs clearly showed that there was a true synergism between WGHE and SDS. There is a strong synergistic inhibition effect of WGHE and SDS on the corrosion of CRS in 1.0 mol/L H3PO4 solution, and the inhibitive action of the mixture of WGHE and SDS drastically strengthens, which retards both cathodic and anodic corrosive reactions simultaneously.