Abstract: In order to investigate the mesoscopic mechanism of thaw settlement for soft soil, freeze-thaw（F-T） test and X-ray computed tomography（X-CT）, combined with image processing technology and fractal theory, were adopted to study the changes of meso-structure for silty clay in Shanghai before and after F-T with or without water supply during freezing. Results showed that, the freeze-necking phenomenon was observed in soil specimen without water supply, but it did not happen in soil specimen with water supply. There was a good linear relationship between the change of the average CT gray intensity after F-T and the changes in soil moisture content, porosity, and dry density. Based on the proposed soil mesopore recognition method, it was found that freeze-thaw significantly changed the transverse-sectional porosity and average mesopore size, and the biggest change happened in unfrozen area with water supply, but happened near the final freezing front without water supply. Fractal dimensions had better linear relation with transverse-sectional porosity and average mesopore size, respectively. The study showed that the change in soil meso-structure with water supply during freezing differed significantly from that without water supply during F-T, and F-T altered soil mesopore structure obviously under water-supply condition.