To improve the corrosion inhibition performance of inhibitors in acid solution, we constructed mild steel (Fe) surface models containing 2-Mercaptobenzothiazole (MBT) and different concentrations of chloride ion (Cl–) in sulfuric acid (H2SO4) solution, and carried out the molecular dynamics calculations. Adsorption configurations of inhibitor molecules on the metal surface were monitored to investigate the densification behavior of inhibitor film at different Cl– numbers; the stability of the inhibitor film was evaluated by extracting the mean square displacement (MSD) curve of inhibitor molecules; the density distribution and diffusion coefficient of water molecules were calculated to investigate the water displacement ability of the inhibitor film and water molecule migration ability inside the inhibitor film, respectively. The results show that there is a synergistic effect between MBT and Cl–. When the Cl– number increased from 0 to 25, the thickness (0.42~1.51 nm) and densification (occupied area 1.31~0.33 nm2) of MBT film increased, the self-diffusion coefficient of inhibitor molecules decreased (from 1.25×10–9 m2/s to nearly 0 m2/s), the adsorption peak intensity (relative density 72.3~33.9 nm–3) and diffusion coefficient (1.495×10–9~0.627×10–9 m2/s) decreased. However, there is an opposite trend when the Cl－number increased from 25 to 36. Thus, there is a synergistic effect between the inhibitor molecules and Cl–, and an appropriate concentration of KCl can improve the inhibition performance of MBT in H2SO4 solution. In our work, MBT inhibitors possess the highest inhibition efficiency when Cl– number is 25.