<p>To address the severe issue of subgrade frost heave in season-frozen soil highway engineering in the Qinghai-Tibet Plateau (QTP) region, this studied investigated the frost-heaving characteristics (FHC) of subgrade soil with different initial water content (IWC) and fine particle content (FPC) under unidirectional freezing (UDF) and surrounding freezing (SDF) conditions through laboratory experiments. Furthermore, based on the thermo-hydro-mechanical (THM) coupling theory, variations in the temperature, moisture, and stress fields were analyzed numerically. The findings aim to provide a reference for the development of preventive and control measures against subgrade frost heave. The results show that the frost-heaving deformation (FHD) of subgrade soil can be divided into four stages: frost-shrinkage deformation stage, FHD rapid increase stage, FHD slow increase stage, and FHD stabilization stage; A sensitivity analysis suggests that controlling the IWC and FPC at approximately 8% and 9%, respectively, in the subgrade gradation design can effectively inhibit frost heave. Compared to the SDF condition, the UDF condition resulted in a more significant temperature gradient and a distinct freezing front. However, stress concentration points were observed under both freezing conditions</p>