Abstract: In view of the corrosion problems that may be caused by Cl^- and SO₄^2- in feed water or boiler water of high parameter units, the effects of Cl^- and SO₄^2- on the corrosion of 15CrMoG steel were investigated by 24 h coupon corrosion tests for 15CrMoG steel in water containing different content of Cl^- and SO₄^2- under simulated oxygenation condition (300℃, 8.5 MPa), and by analysis with atomic absorption spectrophotometer, metallographic microscope, scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD). The results showed that both Cl^- and SO₄^2- damaged the Fe₃O₄ passivation film on 15CrMoG steel surface, causing pitting corrosion and promotion of Fe dissolution. Cl^- and SO₄^2- showed obvious synergistic effect when two ions existed at the same time, which resulted the increase of pits in quantity and depth and the increase of Fe ions in solution.