The effect of ClO₂^- amount on the metastable pitting of S2205 stainless steel in Cl^- containing medium was studied by means of slow potentiodynamic scan. The results show that the competitive adsorption of ClO₂^- and O₂ hinders the formation and repairation of the passivation film, while the strong oxidation causes the partial dissolution of the passivation film, thereby its stability is reduced. Meanwhile, ClO₂^- competes with Cl^- for adsorption, the greater the concentration, the higher the replacement rate, as a result, such competition may provide protectiveness for the passivation film to a certain extent. The competitive adsorption of ClO₂^- and Cl^- can suppress the metastable pitting nucleation, at this time, the nucleation could mainly occur on inclusions and active sites on the metal surface, of which the later might be generated on the surface grooves with a certain critical depth. Due to the decrease in the stability of the passivation film, as the concentration of ClO₂^- increases, the number of metastable pitting nucleation increases slightly, and the growth rate of metastable pitting has little relationship with the concentration of ClO₂^-. After the addition of ClO₂^-, Cl^- induced metastable pitting nucleation was inhibited, E_m increases rapidly, then its change with the concentration is not obvious, and the stability of the passivation film decreases, caused E_b decreases rapidly, it slowly decreases with the increasing concentration of ClO₂^-, then the metastable state of pitting is more likely to be transformed into the stable stage. The increase in potential could facilitate the dissolution of inclusions on the metal surface, inhibiting the metastable pitting nucleation, while increasing the Cl^- migration rate and thus promoting the growth of metastable pitting.