Abstract: The effects of vapour on water content in different unsaturated frozen soils have not been systematically analyzed in the literatures. Based on the thermodynamic equilibrium theory and coupled water-heat theory, a new method for calculating unfrozen water content and ice content is obtained. A new model is then established by importing this method to the coupled heat and mass transfer theory. The unfrozen water content and ice content in this new model are only related to the hydraulic parameters and temperature, which have specific physical meanings. The comparisons between the simulations and the test results of sandy loam validate the new model. The simulated results also show that the temperature is the major factor to vapour transfer instead of the suction. And the vapour transfer in silt and sand cannot be neglected with freezing except clay. The initial water content, freezing temperature, freezing time and ground water table can all affect the vapour transfer in freezing soils. In a word, even though the water content increament is low, remarkable frost heave will also occur due to the vapour in susceptible frost heaving soils such as silt. Therefore, the vapour in unsaturated frozen soils must be paid more attention to in practical engineering. This study strengthens the understanding of canopy effect and also validates that the canopy effect usually occurs in covered freezing silt instead of sand or clay.