Abstract: The unfrozen water content of weakly expansive soil foundations during freezing is one of the important factors to be considered in the design of a new type of 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 uses a low-field nuclear magnetic resonance (NMR) system to test its unfrozen water content during freezing (+15℃~-30℃), analyzes the effects of temperature, swelling value and initial water content on its pore characteristics and unfrozen water content, reveals its freezing mechanism, and establishes a prediction model for weakly expansive soil's unfrozen water content. The results show that during freezing, water in macropores of weakly expansive soil freezes first, followed by that in micropores; porosity decreases stepwise, and a sharp water-ice phase transition inside the soil at -2℃~-4℃. In the freezing stage, the larger the swelling value the lower freezing point, the higher the unfrozen water content in micropores/mesopores and lower macropores; the higher the initial water content, the higher freezing point and unfrozen water content, with the unfrozen water in macropores at -6℃. The swelling value, initial water content and temperature are all positively correlate with unfrozen water content. An unfrozen water content for weakly expansive soil is established, combining laboratory tests with external data, its rationality is verified.