The penetration of chloride ion leads to the corrosion initiation in reinforced concrete, which results in decreasing the durability of concrete. A theoretical diffusion-convection model describing the process of chloride ion penetration into concrete under external water pressure is described by considering multiple affecting factors such as unsaturated flow, fluid-solid coupling, and chloride binding. A numerical model of unsaturated concrete is built to simulate the coupled process. Based on this model, the classic expression of effective diffusion coefficient is modified by considering constrictivity factor, and the sensitivity analysis is carried out on five sets of parameters (i.e., effective diffusion coefficient, saturated permeability, van Genuchten parameters, initial saturation, and binding capacity parameters) aiming at evaluating the robustness of the model. The simulation results show that the multi-mechanism penetration model is computationally feasible, and the multiphysics coupling model can well reproduce the chloride ion transfer process in a microscopic perspective. Furthermore, the sensitivity analysis results indicate that the parameters governing moisture transport process are more sensitive to the prediction of the chloride ion penetration into undersea tunnel concrete.