The moisture-vapour-salt (MVS) transfer is the key mechanism driving moisture and salt accumulation beneath impermeable layers in saline soils of cold regions. This process initiates salt-frost heave (SFH), posing serious risks to the safety and stability of roads, hydraulic structures, and power transmission facilities. Through theoretical analysis, the critical factors controlling ice-water phase transition and salt crystallization during freezing in sulfate-affected soils were identified. The dominant influencing parameters were determined, and corresponding calculation methods were proposed. A unidirectional stepwise freezing experiment was conducted on unsaturated sulfate-affected soils to investigate the MVS transfer and spatial redistribution. The mechanisms by which vapour-moisture-ice transitions and salt crystallization contribute to the SFH were clarified. Results show that under stepwise freezing, volumetric unfrozen water content (VWC) and pore solution concentration decrease in distinct stages. In the initial cooling phase, rapid freezing occurs at the cold end, with limited moisture and salt migration. Vapour migrates upward and condenses into a thin ice layer at the surface. As stepwise freezing progresses, continuous MVS transfer toward the freezing front leads to the formation of three distinct freezing regions, each showing peak moisture and salt concentrations near the front. In the unfrozen zone, upward migration of moisture and salt persists until the water content drops below a critical threshold, disrupting capillary continuity. At this point, moisture transport weakens, and soil deformation gradually stabilizes. The SFH is closely associated with the MVS migration, multiphase transitions among air, water, and ice, and salt crystallization. These coupled processes lead to dynamic redistribution and ultimately trigger cascading failures through crystallization-induced deformation. This study reveals the multiphase-multifield coupling mechanism in unsaturated saline soils, providing a theoretical basis for controlling the MVS transfer and preventing SFH in engineering projects in cold and arid regions.