According to the fact that of some water in soil cannot be frozen due to distribution of cationic diffusion layer on the surface of negatively charged clay particles

the theoretical formula of unfrozen water content is obtained by electrical double-layer theory based on Poisson equation of electrostatic field and static charge Boltzmann distribution. the empirical formula of classical unfrozen water content is almost identical to the proposed model. This paper describes the structure and causes of unfrozen water on the surface of soil particles

and quantitatively analyzes the effect of soil type

salinity and temperature on unfrozen water content.When the soil salt concentration is less than a threshold value

changes in salinity negligibly influence the unfrozen water content.This indicated different types of frozen soil texture（low salinity） have analogous structure of electric double layer. the empirical formula of exponential form can describe all unfrozen water content of low salinity frozen soil. The unfrozen water content is dependent on soil types

specific surface area. More clay particles in soil and larger specific surface area resulted in more unfrozen water content. Two existing empirical formulas have been used to verify the theoretical formula. Results prove that the theoretical formula for calculating unfrozen water content in frozen soil is reliable.