The sunny-shady slopes effect is a phenomenon that impacts the temperature distribution of high-speed railway subgrades, resulting in uneven frost heaving deformation on the subgrade surface, which in turn causes static irregularity in the slab track. Based on the hydraulics theory, a thermal-hydro-mechanical (THM) coupled model of frozen soil is established and verified. We explore the process and characteristics of the temperature field and deformation of soil during the freezing process of high-speed railway subgrades and analyze the track irregularity variation law of China Railway Track System III slab tracks under uneven frost heaving deformation. The results show that, because the left and right slopes of high-speed railway subgrade are exposed to different amounts of solar radiation, which is the key factor causing uneven frost heaving of subgrade. Different strike angles cause changes in temperature of the subgrade’s upper part and the frost heaving amount on the surface, leading to differences in the deformation of the slab track structure: Increased strike angle weakens the rail level irregularity of the down line and marginally increases the rail level irregularity of the up line, and these become consistent in north-south directions. Therefore, when selecting railway lines in seasonal frozen areas, the west-east direction should be avoided to prevent the extremes in sunny-shady slopes effect on subgrades.
