<p>The freezing of unsaturated saline soil is a dynamic water–heat–vapor–salt–mechanics coupling process. Salt–frost heave, resulting from water–vapor–salt transfer, poses a significant threat to the stability and reliability of geotechnical engineering in salinized cold regions. Based on Gibbs free energy theory, a theoretical framework incorporating osmotic and matric potentials for calculating relative humidity was proposed, highlighting the role of solutes in water–vapor transfer. Unidirectional freezing experiments were conducted to explore how salt content, water content, temperature gradients, and freezing modes influence water–heat–vapor–salt–mechanics coupling interaction. The results reveal the coupling mechanism of water–vapor–salt migration, heat transfer, phase transformations between water, vapor, and ice, salt crystallization, and salt–frost heave in freezing unsaturated saline soil. The findings show that vapor diffusion is the primary factor driving moisture accumulation beneath the impermeable layer. Solutes in the pores lower relative humidity, slow the water–vapor phase transition, and hinder vapor diffusion. Water redistribution is influenced by the spatiotemporal variations in water and vapor transfer rates, with a critical moisture threshold required to enhance vapor migration. Below this threshold, vapor transfer becomes significantly more intense. Near the freezing front, overlapping peaks of water and salt concentration create a new impermeable layer due to the accumulation of ice and salt crystals. This process further intensifies water–vapor–salt migration, amplifying salt–frost heave. These findings provide crucial insights into the dynamics of water–vapor–salt interaction and offer strategies for mitigating salt–frost heave in salinized cold regions.</p>