The slag resistance of ladle lining refractory in different atmospheric conditions is of utmost importance due to the strong variation of atmospheric environment in ladle during the process cycle. In this study, by adopting dynamic induction furnace corrosion test, the corrosion mechanism of lightweight Al2O3-MgO castable with different environmental oxygen partial pressure was investigated through macro and microanalysis, XRD, and thermodynamic simulation. The atmospheric condition was set to P (O-2)=0.21atm or P(Ar)=1.0atm. The attained results showed that a reduced slag corrosion but intensified slag penetration happened at low environmental oxygen partial pressure condition. With P(O-2)=0.21atm, Mn and Fe in slag were present in the form of divalent and/or trivalent cations and were incorporated into spinel to form MnFe2O4 solutions and MgAl2O4 solutions during the corrosion process. Since Fe, Mn ions were largely consumed, liquid with high viscosity formed and the continuing infiltration was suppressed. Under P(Ar)=1.0atm, Fe ions in the slag were totally reduced into elemental iron while Mn existed mainly in the state of MnO, and the amount of liquid slag was therefore reduced, leading to significantly weakened corrosion on castable aggregates. Corrosion reaction products under this condition were mainly MgAl2O4 and CA(6).