The corrosion behavior of 16Mn steel was studied in saturated H2S or H2S/CO2 solutions containing different Cl− concentrations at 80 °C. The microstructure and chemical composition of the corrosion products were investigated through scanning electron microscopy, energy-dispersive X-ray spectroscopy, EPMA, and X-ray diffraction. Results showed that the corrosion rate decreased with increasing Cl− concentration in saturated H2S or H2S/CO2 solution at pH 4. Conversely, the corrosion rate increased with increasing Cl− concentration in saturated H2S solution at pH 6. The relative H+ concentration decreased because of the increase of Cl− concentration at pH 4, and Cl− acted as a catalyst in the corrosive medium at pH 6 because the net H+ concentration decreased obviously compared with the condition at pH4. Cl− promoted the formation of Fe-deficient iron sulfide at pH 4, and the opposite effect was observed in the nearly neutral solution. The corrosion rate increased firstly with increasing Cl− concentration and then decreased in the saturated H2S/CO2 solution at pH 6. The corrosion products were mainly composed of two kinds of iron sulfide. Sulfide FeS1−x was a kind of tetragonal crystal, whereas the other was the hexagonal/monoclinic iron sulfide Fe1−xS. The corrosion film that was mainly composed of FeS1−x did not confer a protective effect on the base metal. The atomic ratio of Fe/S was more than 1 for FeS1−x. The appearance of sulfide FeS1−x resembled a square block or small, needle-like, flocculent particles. The atomic ratio of Fe/S was less than 1 for Fe1−xS, and the corrosion film mainly composed of Fe1−xS conferred some protective property on the base metal. The sulfide FeS1−x exhibited a long claviform morphology with a hexagonal or quadrilateral cross-sectional shape.