The corrosion behavior of as-cast GH3625 alloy were studied in different corrosive media (Air、75 wt% Na₂SO₄+25 wt% NaCl and 2% SO₂+H₂O+Air) at 900 ℃ for 120 h, utilizing XRD, SEM, EDS, weight-gain measurement and thermodynamic calculation. The formation and destruction mechanism of the oxide film under different corrosive media were discussed. Results show that from the top layer to the internal structure of GH3625 alloy is divided into loose oxide layer, dense oxide layer, Cr poor zone and alloy matrix after corrosion in different media, which is mainly composed of NiO, Cr₂O₃ and NiCr₂O₄. In addition, the depth of Cr-poor zone after corrosion in corrosive medium (14μm and 11μm) is greater than that after oxidation in air (8μm). The damage degree of corrosion medium to the surface oxide film of the alloy is molten salt medium, acidic atmosphere medium and air medium in descending order. The corrosion of GH3625 alloy is mainly due to the competition between the formation and dissolution of the surface oxide film in different media. When GH3625 alloy is corroded in 75 wt.% Na₂SO₄+25 wt.% NaCl medium, the corrosion mechanism is mainly that the oxide film reacts with Cl^_- to form gas phase Cl₂ and reacts with Na₂O to form chromate Na₂Cro₄. However, when GH3625 alloy is corroded in 2% SO₂+H₂O+Air, the corrosion mechanism is mainly the continuous sulfurization and oxidation reaction of Cr, Ni and SO₂ and O₂.