<p>The pot cover effect can induce various forms of distress in cover layer engineering, such as salt heave, cracking, and differential settlement, with water vapor migration being the primary cause. However, current research on the pot cover effect in saline soils rarely takes into account the water vapor transport process. Therefore, elucidating the coupled transport mechanisms of water, vapor, heat, and salt in saline soils under this effect is crucial for the prevention and control of related engineering hazards. This study developed a numerical model describing the coupled water-vapor-heat-salt transport in unsaturated saline sulfate soil and validated its reliability through laboratory unidirectional freezing column tests. Based on this model, a numerical analysis was conducted to investigate the formation mechanism of the pot cover effect during the unidirectional freezing of the saline soil. The results indicate that the moisture and salt fields exhibit a typical bimodal distribution pattern, with peaks located at the soil surface and the freezing front, respectively. Compared with the initial water content of 19 % and initial salt content of 1 %, the total water content at the surface and freezing front increased by 21 % and 13 %, respectively, while the total salt content rose by 1.25 % and 0.5 %, respectively. Liquid water flux upward in the unfrozen zone, while it approaches zero within the frozen zone. In contrast, both vapor flux and solute flux migrate upward throughout the entire soil column, reaching their maximum values at the freezing front. Compared to models that neglect vapor transport, the simulated total moisture content at the surface was 12 % higher in the model accounting for vapor movement, indicating that water vapor migration is a key factor contributing to moisture accumulation at the surface. The findings of this study can provide a theoretical basis for preventing engineering hazards associated with the pot cover effect in saline soils.</p>