<p>The reduction on the soil temperature by extreme low temperature in natural or artificial cooling methods can prompt a transformation in the existing form of water in soil and further a change in soil properties. This study is inspired by the interesting phenomenon of increased volume in frozen soils before the free water phase transition in a closed environment test. The influence mechanism of pore air pressure on the existence form of water in the soil is revealed and the viewpoint of converting bound water into free water before the free water phase transition has been proposed based on the hypothesis, theoretical analysis, numerical calculations, and experiments. The main research findings are as follows: 1) the first time discovery of the volume increment before the freezing phase transition of free water; 2) the calculation on the value of pore air pressure that can cause the transformation of the water existence form in the soil, i.e., the transformation of bound water into free water; 3) determination on the correlation between the temperature field and the initial water content. This research, which has never been investigated from this aspect, could provide an expansion on the freezing mechanism and freezing theory of soil.</p>