Unfrozen water contents of saturated soil with different soil types and different concentrations of NaCl are measured by low-field nuclear magnetic resonance (NMR) in freezing-thawing cycles. The distribution of unfrozen water in soil is discussed combining with T2 distribution curve from the microscopic viewpoint during freezing and thawing process. It is concluded from the experimental results that the freezing process can be divided into three stages: super-cooling stage, rapid decline stage and stable stage; while the thawing process can only be divided into two stages: stable stage and rapid melting stage, and there is no overheating phenomenon in thawing process. During the freezing process, water in large pore freezes firstly. In a sharp contrast, the pore ice in small pores melts firstly. Thermodynamic potential difference of pore water results in the sequence of phase change of pore water during the freezing-thawing process. In addition, the influence of soil type and ion concentration on unfrozen water and hysteresis during freezing-thawing process is analyzed and possible mechanisms for the hysteresis are discussed.