Abstract: Investigates the redox reaction mechanism of iron as an auxiliary catalyst component for the oxygen electrode in acid environment, and calculates its diffusion coefficient by cyclic voltammetry. Auxiliary catalyst iron for oxygen electrode in acid environment occurs multi electron consecutive reactions, controlled by ion diffusion. The reaction Fe(Ⅲ)→Fe(Ⅱ) has good reversibility, and reaction Fe(Ⅱ)→Fe is considered as quasi reversible, as are apparent from the cyclic voltammograms. Making use of the reversible reaction characteristics, Fe(Ⅲ) diffusion coefficient is obtained to 6 81×10 -6 cm 2/s. The experimental value is in good agreement with the theoretical value.