Imidazoline thiourea oleate (TAI) is a very good corrosion inhibitor for carbon steel in CO2 corrosion environment, however its corrosion inhibition performance in CO2/H2S co-existence system is poor. In order to improve the corrosion inhibition effect for X65 steel in CO2/H2S corrosion environment, Propargyl alcohol is used to modify TAI to obtain a new corrosion inhibitor-dipropargyl methoxy thioureido-imidazoline (DPFTAI). In this article, the inhibition effect and mechanism of TAI and DPFTAI is studied by dynamic weight loss corrosion experiment and electrochemical tests, and the results shown the inhibition efficiency of DPFTAI is significantly improved. Material Studio 7.0 software is employed to calculate the molecular orbital energies of TAI and DPFTAI, adsorption energy of inhibitor, Fukui index, surface interaction force and adsorption state, the interaction between corrosion inhibitor molecules and corrosive ions, etc. The experiment results show that the inhibition efficiency of TAI and DPFTAI are both higher than 93% in the absence of H2S. When the H2S content is 2000 mg/L, the inhibition efficiency of DPFTAI is still as high as 91.96%, which is 18% higher than that of TAI. DPFTAI has three adsorption centers, namely two N atoms on the imidazoline ring and S atoms on the side chain of the molecule. The interaction energy between DPFTAI and iron surface is 29.66 kcal/mol. While TAI has only one adsorption center, which is S atom on the molecule, and its interaction energy with iron surface is 22.46 kcal/mol. The HS– concentration around CO2/H2S environment with DPFTAI inhibitor is significantly higher than that of TAI inhibitor, which further proves that DPFTAI has dramatically better adsorption effect and corrosion inhibition effect on carbon steel than TAI in CO2/H2S corrosion system.