Concrete is an anisotropic material composed of aggregate, mortar, and interface transition zone(ITZ). Assuming that the interface transition zone thickness follows a normal distribution, a two dimensional concrete random aggregate model is developed. On this basis, a mesoscopic numerical model for predicting the service life of marine structure is established by coupling chloride ion diffusion, steel corrosion and concrete damage. The numerical simulation of chloride ion content distribution and crack morphology are in good agreement with experimental data, and verifies the reliability of the numerical model. Numerical studies show that chloride ions cause non uniform corrosion of single steel bar, resulting in three main cracks in the protective layer. The angle between the three main cracks is about 120°. The thickness of the interface transition zone has a critical impact on the service life of the marine structures. The main factor leading to this phenomenon is that the interface transition zone accelerates the diffusion of chloride ions, and the secondary factor is the deterioration of the mechanical properties of the interface transition zone. Under the condition of the same amount of steel bar corrosion, the service life of reinforced concrete with non uniform corrosion is shorter than that with uniform corrosion, and the cracking pattern of the protective layer is also different.