Abstract: Horizontal freezing heave induces frost heave pressure in retaining structures, foundation pits, and slopes under various conditions, resulting in a range of engineering challenges. Building on the existing thermo-hydro-mechanical (THM) coupled frost heave model for saturated soils, a THM coupled model for horizontal freezing in unsaturated soils is developed, integrating vapor migration and ice segregation mechanism. The fundamental equations consider soil skeleton deformation, water flow, and heat transfer, while also describing the mechanisms of ice lens formation and growth. Numerical simulations are performed using the COMSOL Multiphysics platform. The reliability of the proposed model is verified by comparing the simulation results with those from horizontal frost heave tests. The results indicate that higher initial water content significantly enhances soil frost heave, while higher dry density inhibits ice lens development by slowing water migration. Additionally, the findings reveal the coupling mechanism between frost heave pressure and soil deformation field, providing key theoretical support for understanding horizontal frost heave.