Abstract: In order to study the effect of moisture content and fiber content on the frost heaving and thawing settlement of subgrade soil improved by fiber under freezing and thawing cycles, the indoor unidirectional freezing and thawing tests were carried out, the changes of moisture field, frost heaving rate, thawing settlement coefficient, temperature field and freezing temperature in the whole process of freezing and thawing under different moisture content and fiber content were obtained. The results showed that: 1) during the unidirectional freezing and thawing process of the sample, the water phase transition became smaller and smaller with the increase of the depth, and the water content was not consistent after the freeze-thaw process. The moisture redistribution of soil samples with different initial moisture content after freezing-thawing cycles was obvious, and the moisture distribution of soil samples with different fiber content after freezing-thawing cycles was basically similar. With the increase of fiber content, the degree of water migration decreases. 2) In the range of 8%-16% moisture content, the maximum vertical displacement of freezing reached1.19 mm, and the maximum vertical displacement of thawing reached 2.56 mm. When the moisture content reached 12%, the frost heave rate and thawing settlement coefficient tended to be stable. In the range of 0%-0.5% fiber content, the maximum vertical displacement of freezing reached 1.10 mm, and the maximum vertical displacement of melting reached 2.85 mm. 3) with the increase of fiber content, the latent heat of phase transformation became more obvious, and the soil freezing temperature first decreased and then increased. The lowest freezing temperature was-1.95 ℃ when the fiber content was 0.3%, and the longest time to reach the freezing temperature was 14.1 min. The results of this study can provide reference for railway and engineering construction in silty sand areas in the west of China.