Objective To improve the corrosion inhibition performance of phytic acid for hot rolled carbon steel (HRCS) in 0. 5 mol/ L HCl soltuion. Methods The corrosion inhibition efficiency of phytic acid, Tween-40, and their compound on carbon steel was measured by weight-loss method and electrochemical impedance spectrometry tests. The inhibition mechanism of the compound inhibitor was analyzed using thermodynamics and kinetics. Results It was revealed by weight-loss measurement that either phytic acid or tween-40 alone had limited inhibitory effect on the corrosion of HRCS in hydrochloric acid. The inhibition efficiency of phytic acid reached the max value of 69. 1% when the concentration was 0. 3 g/ L at 293 K, then decreased with further addition of inhibitor and was reduced to 9. 6% when the concentration of phytic acid was 0. 8 g/ L. On the other hand, the inhibition efficiency increased with the increase of the tween-40’s concentration, which reached the value of 73. 4% at 0. 8 g/ L, 293 K. When Tween40 (0. 05 g/ L) was added into the solution, the inhibition efficiency of the compound inhibitor increased even if the concentration of phytic acid exceeded 0. 3 g/ L. The corrosion inhibition efficiency of compound inhibitor was higher than the sum of phytic acid’ s and Tween-40’s efficiency, which suggested synergistic effect. The same result could be concluded from electrochemical impedance spectrometry measurement. Further analysis revealed that the adsorption of the compound inhibitor on the surface of carbon steel obeyed Langmuir adsorption isotherm equation, which improved the activation energy(Ea) of the corrosion procession, leading to the decrease of entropy. Conclusion Tween-40 and phytic acid showed synergistic corrosion inhibition effect on the HRCS in hydrochloric acid.