Candidate　materials　of　supercritical　and　ultra-supercritical　utility　boiler　water　wall　T91　and　12Cr1MoV　were　chosen　as　the　experimental　samples,　and　exposed　to　an　oxidizing　atmosphere,　a　reducing　atmosphere　and　an　oxidizing/reducing　alternating　atmosphere　respectively.　The　corrosion　temperature　was　450°C.　The　corroded　samples　were　analyzed　by　scanning　electron　microscope　and　energy　dispersive　spectrometer.　To　describe　the　corrosion　of　the　materials　accurately,　the　mass　gain,　the　macro-thickness,　and　the　migration　depth　were　simultaneously　analyzed.　The　combination　corrosion　of　oxygen　and　sulfur　had　a　stronger　corrodibility　than　that　of　pure　H2S.　Cr2O3had　little　effect　in　resisting　H2S　corrosion,　but　it　can　hinder　the　combination　corrosion　model.　T91　had　a　stronger　corrosion　resistance　than　12Cr1MoV.