Abstract: Unfrozen water content of weakly expansive soil foundations during freezing is a key factor to be considered in the design of new type high-speed railway cutting pile-slab structures. Taking the weakly expansive soil from the foundation of the Lanzhou-Xinjiang High-Speed Railway as the research object, this study used a low-field NMR system to test its unfrozen water content during freezing (+15℃~-30℃), analyzed effects of temperature, swelling value and initial water content on its pore characteristics and unfrozen water content, revealed its freezing mechanism, and established a prediction model for weakly expansive soil’s unfrozen water content. Results show: during freezing, water in macropores of weakly expansive soil freezes first, then that in micropores; porosity decreases stepwise, with drastic water-ice phase transition inside at -2℃~-4℃. In the freezing stage, larger swelling value brings lower freezing point, higher unfrozen water content in micropores/mesopores and fewer macropores; higher initial water content leads to higher freezing point and unfrozen water content, with macropore unfrozen water fully frozen at -6℃. Swelling value, initial water content and temperature all positively correlate with unfrozen water content. The unfrozen water content model was verified rationality via error analysis and fitting coefficient calculation based on laboratory tests and external data.