Density functional theory (DFT) calculation and molecular dynamics (MD) simulation were performed to do an in-depth study on the inhibition mechanism of quaternary ammonium surfactant CI molecules with a different chain length in the presence of 1.0 M HCl and 500 ppm acetic acid on the Fe (110) metal surface. Results from DFT calculation showed that all surfactant CI molecules have good inhibition properties where the cationic quaternary ammonium groups (N⁺) and the alpha carbon act as a reactive centre to donate electrons to the metal surface with low band-gap energy of 1.26 eV. In the MD simulation, C12 with a 12-alkyl chain length showed the most promising CI molecules with high adsorption energy and binding energy values, low diffusion coefficient towards the corrosion particles and randomly scattered at low concentration that give better adsorption towards the Fe (110) metal surface. The finding on the effect of the alkyl chain length on the inhibition efficiency of all quaternary ammonium CI molecules based on computer modelling data and the success of an in-depth study on the theoretical understanding of quaternary ammonium surfactant CI molecules in the acidic medium corrosion system towards metal surface could be used as the future development of new surfactant CI molecules with ammonium-based functional groups.