A kind of efficient, environment-friendly compound inhibitor was prepared with 2-mercaptobenzimidazole (2MBI), potassium iodide, triethanolamine, polyoxyethylene nonylphenyl ether and peregal by weight loss through orthogonal experiment. The effects of the compound on corrosion of A20 steel in citric acid have been investigated in relation to it’s concentration by weight loss measurements, polarization curve methods and electrochemical impedance spectroscopy (EIS). The result of weight loss reveals that the compound is a good inhibitor whose efficiency can reach 98.8%. Polarization curve results show that the compound restrain the process of cathode and anode, belonging to anode control-oriented and charge transfer-controlled. EIS results show that the changes in the impedance parameters (Rp and Cd) with concentrations of the compound are indicative of the adsorption of these molecules leading to the formation of a protective layer on steel surface.