In order to accurately simulate the diffusion of chloride ion in the existing concrete bridge and acquire the precise concentration of chloride ion at any time, a Cellular Automata(CA)-based model is proposed. Firstly, the process of chloride ion diffusion is analyzed by the CA method and a nonlinear solution to the Fick’s second law is obtained. Secondly, considering the impact of various factors such as stress states, temporal and spatial variability of diffusion parameters and water-cement ratio on the process of chloride ion diffusion, the model of chloride ion diffusion under multifactor coupling is presented. Finally, a chloride ion penetrating experiment is used to prove the effectiveness and reasonability of this present method. A T-type beam is taken as an illustrative example to analyze the process of chloride ion diffusion.