Through electrochemical methods and weight loss measurement, the corrosion inhibition of ficus microcarpa leaves extract ( FLE) for carbon steel in sulfuric acid solutions was studied, and the synergistic effect between FLE and KI was investigated, furthermore the inhibition mechanism of FLE was discussed. The results showed that FLE was a mixed-type inhibitor for carbon steel,and the inhibition efficiency of FLE for carbon steel increased with increase of the concentration of FLE.There existed significant synergistic effect between FLE and KI,which made the mixture of FLE and KI was used as corrosion inhibitor,the corrosion current density and double layer capacitance decreased, the charge transfer resistance increased, consequently showed better corrosion inhibition and higher inhibition efficiency. The adsorption of FLE with the presence and absence of KI on the carbon steel surface was a spontaneous process,and obeyed Langmuir adsorption isotherm model .