The corrosion characteristics of 15CrMoG steel a supercritical unit water-wall tube material in two kinds of waters were studied by means of metallography, SEM/EDS, and XRD etc. The two waters aim to simulate the boiler water after the all-volatile treatment (reduction) and the oxygenated treatment, i.e. all-volative treatment (reduction) (ATV(R)) water and oxygenated treatment (OT) water respectively. The results showed that a much uniform oxide scale with much uniform particles size distribution grew on the steel surface under the AVT(R) condition rather than that under OT condition. The corrosion degree of the steel increased in the OT water with addition of Cl^- or Cl^-/SO₄^2- with increasing ion concentrations. At the same ion concentration, the corrosion of the steel under the simulated OT condition was more severe than that under AVT(R) condition and the Cl^-/SO₄^2- was more corrosive than Cl^- under the same condition. Due to autocatalytic effect of pitting corrosion, serious local corrosion occurred on the steel in the simulated AVT(R) and OT waters. It is noted that Cl^- was not detected but an obvious S content was detected by EDS in the pits on the steel surface under the two conditions with different ion in waters.