The work aims to study the synergistic effect of alanine and KI coexisting in H2SO4 solution on corrosion inhibition of carbon steel. The anticorrosion performance and adsorption mechanism of alanine and complex inhibitor of alanine and KI on carbon steel were studied with dynamic polarization curves, electrochemical impedance spectroscopy, SEM, X-ray photoelectron spectroscopy (XPS) and El-Awady dynamic model. The anticorrosion performance of corrosion inhibitors on carbon steel improved with increasing of concentration in the 10% H2SO4 solution. When the alanine was used as corrosion inhibitor alone, the adsorption of the alanine molecular was monolayer, the inhibition efficiency was only up to 29% and the corrosion inhibition effect was not significant. However, when the alanine and KI coexisted in the solution, the corrosion inhibition effect was improved remarkably. When the mass concentration of alanine was 300 mg/L, and KI mass concentration was 250 mg/L, the corrosion inhibition efficiency could reach about 92%. XPS results showed that the corrosion inhibitor was mainly adsorbed on the surface of carbon steel by forming covalent bonds between N and Fe atoms. When KI was added, I- was adsorbed on the metal surface and oxidized to I3- partly. The El-Awady dynamic model indicated that the compounded corrosion inhibitor was mixed type and the adsorption process was multi-molecular layer adsorption. In 10% sulfuric acid solution, alanine molecules are adsorbed on the surface of carbon steel through physical adsorption or chemical adsorption to slow down corrosion reaction. After added, the KI acts as a bridge connecting inhibitor molecules with the carbon steel surface, thus assisting alanine to adsorb to the carbon steel surface, improving the coverage rate of alanine on the carbon steel surface, increasing the corrosion inhibition efficiency, reducing the use amount of alanine, and effectively inhibiting the corrosion of steel.