Existence of salt ions significantly influences corrosion-inhibitor adsorption on metal surfaces and yields different inhibition efficiency. In this work, adopting molecular dynamics simulations, the effects of salt concentrations on inhibition performances were studied. Simulation results demonstrate that increasing salt concentration leads to initial decrease and subsequent increase of the inhibition efficiency. The competitive adsorption exists between protonated inhibitor and cations on negatively charged metal surface, and the electrostatic screening controlled by dissolved cations and anions are responsible for different adsorption behaviors. Our work provides a deep understanding of salt effect at the atomic level, and consummate the current inhibition theory.