Objective Amino acids are cheap, innoxious, biodegradable and with a great potential for wide application. This work aimed to investigate the corrosion inhibition of Q235 steel in HCl medium by composite amino acid corrosion inhibitor. Methods In order to determine the corrosion rate of metals, the samples were weighed before and after the tests using an analytical balance with a precision of 0.1 mg. The corrosion inhibition performance of L-cysteine and its composite corrosion inhibitor for Q235 steel in 1 mol/L hydrochloric acid medium was analyzed by weight loss method, and the inhibition mechanism was discussed by isothermal adsorption model. Results The corrosion efficiency of L-cysteine was not high when used alone. The corrosion inhibition of the single inhibitor was 65.57% when the dosage was 800 mg/L. The ternary inhibitor could reduce the cost and improve the efficiency of corrosion inhibition. L-cysteine, KI and ascorbic acid were mixed together and the optimal mixture ratio was obtained. When the concentrations of L-cysteine, KI and ascorbic acid were 20 mg/L, 30 mg/L and 250 mg/L, respectively, the corrosion inhibition efficiency reached up to 96.37%. The composite corrosion inhibitor obeyed Langmuir adsorption isotherm on the surface of Q235 steel. Conclusion Composite corrosion inhibitor had good inhibitory effect for Q235 steel in HCl medium at reasonable dosage and cost, and is thus an environment-friendly green inhibitor with wide application prospect.