Abstract: The coupled heat and water transfer in frozen soil presents certain challenges for numerical computation due to the strong coupling nature of its control equations, thereby affecting their application in engineering practice. Based on the principles of energy conservation, mass conservation, and the soil freezing curve, a theoretical model for the coupled heat and water transfer in frozen soil, considering phase change effects, is proposed. Subsequently, decoupled control equations are derived through mathematical deduction to optimize numerical solutions. Numerical modeling of the coupled water and heat transfer process in frozen soil is implemented through secondary development on the COMSOL platform. The measured data from the subgrade of the Lanxin Passenger Dedicated Line were used for numerical validation, and numerical analysis was conducted under fitted surface boundary conditions. The analysis indicates the following: (1) The numerical solutions of temperature and moisture content at different depth exhibit good agreement with the measured values, thereby validating the reliability of the decoupled theoretical model for coupled heat and water transfer in frozen soil. (2) The soil layer has a “peak damping” effect on the amplitude of periodic variations in temperature and moisture content, and the sine wave curves of temperature and moisture content at different depth exhibit certain phase lag phenomena. Among these, temperature amplitude attenuation and phase lag are caused by energy dissipation in the heat conduction process, while similar phenomena in moisture content curves may be due to phase changes between ice and water altering soil permeability. (3) The temperature contour lines near the surface are denser, while those in the deeper soil layers are sparser. The surface layer of the roadbed is more susceptible to external temperature fluctuations. During summer, the temperature gradually decreases from top to bottom, whereas in winter, the temperature gradually increases from top to bottom. (4) The moisture content in the subgrade section increases with depth, reaching a peak near the interface between the cemented coarse-grained soil material and the fill material, which demonstrates the impact of material interfaces on moisture migration, and then gradually decreases with increasing depth. The research findings provide technical support for the construction of engineering projects such as roadbeds in cold regions.