The work aims to explore the corrosion mechanism and corrosion process of 20Cr steel in different sodium aluminate solutions with different S2- concentrations. 20Cr steel was subjected to high temperature immersion corrosion test and electrochemical test in different S2- concentration solutions. Combined with scanning electron microscope (SEM), X-ray energy disperse spectroscopy (EDS), X-ray diffraction (XRD) and other analytical techniques, the microstructure, element content and composition of corrosion products were observed. The corrosion rate of the sample was analyzed by weight loss method, polarization curve and AC impedance map. With the concentration of S2- from 0 g/L to 5 g/L, the corrosion rate increased from 3.43 g/(m2×d) to 25.20 g/(m2×d), and the corrosion product increased gradually. The corrosion rate decreased to 21.30 g/(m2×d) when the concentration of S2- was 6 g/L. The results of EDS showed that with the increase of S2- concentration, the content of chromium in corrosion products increased, the total content of iron decreased, and the content of oxygen and aluminum increased first and then decreased. XRD results showed that the phase composition of the corrosion product changed from Fe3O4 and FeOOH to Fe3O4, FeCr2O4 and FeS after S2- was added. Electrochemical tests showed that when the S2- concentration ranged from 0 g/L to 5 g/L, the corrosion current density increased from 22.87 mA/cm2 to 586.02 mA/cm2, and when the S2- concentration was 6 g/L, the current density decreased to 183.06 mA/cm2. When the concentration of S2- is in the range of 0~ 5 g/L, the corrosion rate of 20Cr steel increases gradually, and FeS is formed in the corrosion product. The addition of S2- can promote the corrosion reaction and destroy the dense corrosive formed when no sulfur element is added. After the S2- concentration exceeds 5 g/L, a dense FeCr2O4 spinel oxide is formed, which adheres to the surface of the sample and inhibits corrosion.