The paper aims to study the mechanism of improving the pitting resistance of Fe-Cr-Ni medium entropy alloy (MEA) in 3.5wt.% NaCl solution by Ce modification. The pitting resistance of Ce-modified and Ca-modified Fe-Cr-Ni MEA were analyzed by potentiometric polarization experiment. The morphology and element composition of inclusions in the alloy with different modification treatments were observed by SEM and EDS, and the effect of RE Ce on the protective force of passivation film was studied by impedance spectroscopy and XPS technique. The morphology of inclusion in Fe-Cr-Ni MEA with different modification treatments was not consistent. Inclusions in Ca-modified alloy could be divided into two categories which were CaO-Al2O3 and CaS-Al2O3 composite inclusions, and the preferential dissolution of CaS in the corrosion process lead to the initiation of pitting corrosion. The inclusions in Ce-modified Fe-Cr-Ni MEA were (Ce,Si)xOy-(Al,Mn)xOy rare earth composite inclusion with a core-shell structure, which reduced the pitting sensitivity and improved the pitting resistance. In 3.5wt.% NaCl solution, the corrosion current density of Ca modified Fe-Cr-Ni entropy alloy was 3.108×10?6 μA/cm2, and the corrosion current density of Ce modified alloy was 4.883×10?7 μA/cm2 which was reduced by 85%. The capacitive reactance arc of Ce-modified and Ca-modified Fe-Cr-Ni MEA showed an incomplete semicircircle shape. The charge transfer resistance between the passivation film and matrix was 1.872×105 Ω.cm2 and 4.312×105 Ω.cm2. The charge transfer resistance in the passivation film of the Ce-modified alloy increased by 1.3 times. RE Ce increased the Cr+Ni/Fe+Mn cation ratio of the passivated film, and the Cr+Ni/Fe+Mn cation ratio of Ce-modified and Ca-modified passivation films was 1.66 and 1.2, respectively. The result of XPS analysis also indicated that the passivation film of Fe-Cr-Ni MEA by Ce modification had better protection. Compared with Ca-modified Fe-Cr-Ni MEA, Ce-modified alloy has better pitting resistance which is mainly attributed to the influence of RE Ce. On the one hand, Ce modifies the inclusion in the alloy and reduces the pitting sensitivity; on the other hand, Ce increases the protection ability of passivation film and improves the pitting resistance.