To study the degradation law of lateral bearing behavior of corroded RC (reinforced concrete) square piles, the governing model of chloride diffusion into RC square pile was established based on Fick’s second law. According to the initial and boundary conditions, the analytical solution of chloride diffusion equation was presented. Based on Faraday’s law, the corrosion depth and the time for corrosion-induced cover cracking in RC square pile were estimated. On this basis, the degradation coefficient of the bending stiffness of the RC square pile was introduced, and the lateral bearing behavior of corroded RC square piles exposed to marine environment was further analyzed. The analysis results illustrated that the chloride-induced reinforcement corrosion had a significant influence on the behavior of laterally loaded RC square piles exposed to marine environments. The bending stiffness of the RC square pile decreased obviously with the increase of the exposed time and deterioration of exposure condition; while the maximum negative shear force, the lateral deflection and the bending moment increased with the increase of the exposed time. Given the same load, the exposed time posed a more obvious impact on the horizontal displacement of pile than on shear force and bending moment. The research findings offer reference for the design of RC pile foundation subjected to marine environment.