Objective In order to provide theoretical data for industrial production,the synergistic corrosion inhibition effect of cetyldimethylethylammonium bromide (CDAB) and NH4SCN for Q235 steel was investigated in 0. 5 mol/ L sulfuric acid solution, and the corrosion inhibition mechanism and performance were explored. Methods Firstly, the relationship between the corrosion inhibition efficiency and the concentration of CDAB was studied by the weight loss method. Then, the corrosion inhibition efficiency and mechanism of the compound inhibitor were analyzed by weight loss measurements, potentiodynamic polarization curves and electrical impedance spectroscopy (EIS). Results The results of weight loss method showed that CDAB had a low corrosion inhibition efficiency when acted alone, and the inhibition efficiency increased with the increasing CDAB concentration. The corrosion inhibition efficiency was only 85. 07% when the concentration of CDAB was 10 mg / L, while the inhibition efficiency increased to 96. 73% when CDAB was compounded with 30 mg / L NH4 SCN. The experimental results revealed that the corrosion of Q235 steel in 0. 5 mol / L H2 SO4 solution was strongly suppressed by the compound inhibitor. The polarization curve results showed that the compound inhibitor used in this paper was a mix-type inhibitor which mainly retarded anodic process, and the inhibition efficiency increased with the increasing CDAB concentration. The anti-corrosion performances determined by the potentiodynamic polarization curve, weight loss method and EIS were almost consistent. The adsorption of the compound inhibitor on Q235 steel surface abided the Langmiur’s adsorption isotherm and the Gibbs free energy of adsorption △G0 was -48. 33 kJ / mol. The adsorption was a spontaneous chemisorption process. Conclusion Composite of CDAB and NH4 SCN showed excellent synergistic effect in the presence of 0. 5 mol / L sulfuric acid solution,and the corrosion of Q235 steel in 0. 5 mol / L sulfuric acid solution was effectively suppressed by the compound inhibitor. The compound inhibitor had a high corrosion inhibition efficiency.