The cathodic electrochemical behavior of A3 mild steel in seawater was studied and the roles of the formed rust layers in cathodic process were analyzed by means of electrochemical tests and XRD. The results showed that the rust formed could be classified into outer layer and inner layer.  The inner layer is mainly composed of γ-FeOOH, α-FeOOH, β-FeOOH and Fe₃O₄/γ-Fe₂O₃, while the main phases in the outer layer changed with the increase of immersion time.  During the initial immersion period of 126 d, the phase is γ-FeOOH, but after about 364 d immersion, the phases are γ-FeOOH, α-FeOOH and Fe₃O₄/γ-Fe₂O₃.  These rust layers play different roles in the cathodic process.  Both the outer and inner rust layers can prevent the dissolved oxygen from diffusing into the steel substrate, while the inner layer could also participate in reduction reaction which accelerated the cathodic reaction.  A parameter α was induced to evaluate the participation degree of rust in the reduction reaction.  The value of parameter $\alpha$ obviously increased during the first 7 d immersion, and almost didn't change after 168 d immersion.  Changes of components in outer and inner layers were discussed and the interactions of different components were analyzed.