The effect of a cathodic inhibitor (3-methylsalicylate, 3-MeSA) on the corrosion properties of Fe-contained pure Mg was investigated. The composition of Fe-rich particles, corrosion surface morphology, and molecular structure of 3-MeSA provide experimental and theoretical evidence for the corrosion behavior of Fe-contained Mg alloy and the corresponding cathodic inhibition mechanism of 3-MeSA. The results showed that silicon in a solid solution within Fe-rich particles was found, and the features of corrosion spots may correlate with different Fe/Si ratios. Although 3-MeSA can form an adsorptive layer on Mg substrate, its inhibition mechanism can be mainly ascribed to the capability to chelate Fe²⁺/Fe³⁺ ions released from self-corrosion of Fe-rich particles, preventing the re-deposition of the reduced Fe species.