Liquid lead–bismuth alloy (LBE) has strong corrosion to metal materials at medium and high temperature, which threatens the long-term safe operation of lead–bismuth cooled reactors. Controlling the concentration of dissolved oxygen in LBE can form a protective oxide film to block further corrosion, but the evolution of oxide layer structure is complex and experimental data are scarce, making it difficult to predict long-term corrosion/oxidation behavior. Therefore, it is of great significance to establish the long-term corrosion/oxidation model of stainless steel in LBE. Based on cellular automata (CA) method, this paper introduces new cellular rules, designs new ion diffusion layer and variable oxygen concentration parameters, and constructs an improved CA model. It is used to study the long-term corrosion/oxidation process and two-phase oxide layer growth of stainless steel under oxygen LBE. The results show that the model can accurately reproduce the whole corrosion/oxidation process, predict the long-term growth trend of the oxide layer.