The characteristic temperatures appearing in freezing process of cooling saline soil include the freezing temperature and the supercooling temperature. A thermodynamically consistent equation for calculating the equilibrium temperature is provided, in which the influences of water activity and pore radius are considered. Since the nucleation in soil can be regarded as the heterogeneous nucleation on a plane surface, an equation for calculation of the supercooling temperature is proposed based on the classical nucleation theory. Through comparing the calculated data with the tested data, the equations are proved effective to predict the characteristic temperatures of saline soil. The effects of solution concentration and the pore size on the characteristic temperatures are discussed. In addition, the influence of the contact angel on the supercooling temperature is investigated. The study would be helpful to understand the freezing mechanism of saline soil and useful for hazard prevention of frost heave.