The passivity of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 (SC-CO2) conditions without or with different partial pressures of O2 was studied by electrochemical measurements and surface characterization. Electrochemical measurements indicate that both the fluid flow and an increase in CO2 pressure lead to a decrease in the stability of the passive film. Introduction of 0.1 MPa O2 into the dynamic SC-CO2 system facilitates the formation of a stable and integrated passive film, which enhances the corrosion resistance of 13Cr stainless steel. However, the addition of 1 MPa O2 reduces the stability of the passive film and results in the formation of a passive film with high donor density, which decreases the corrosion resistance of 13Cr stainless steel.