The chloride ion transport mode and influence mechanism in the coastal dry-wet alternate environment are the main contents of concrete durability research, which has attracted wide attention and discussion of scholars. The calculation model describing the chloride ion transport behavior in concrete is usually composed of partial differential equations, and the solution method is complex. To analyze the chloride ion transport process simply and accurately, this paper proposed a new way to calculate the depth parameter Δx in the modified Fick’s law model. The calculation model of chloride ion transport in the dry-wet alternate region was established by the meso-scale composition of concrete, the ambient temperature field, water migration and chloride ion transport. According to the model, the value of Δx can be accurately obtained. Then the chloride ion transport in different service environments was analyzed. According to the calculation results, the time-varying formula of convection zone depth Δx considering the service time, daily average temperature and drying-wetting time ratio was summarized in this paper. These formulas improve the applicability and accuracy of the modified Fick’s law model for predicting chloride ion transport and distribution in dry-wet alternate environment. The above results can provide a simple and effective calculation method for the durability design of concrete structures in dry-wet alternate environment.