Abstract: Understanding the evolution of cracks under freeze-thaw action is key to revealing the mechanisms of soil frost heave and thaw settlement, which is crucial to guaranteeing the long-term safe service of structures in frozen soil engineering. In this paper, an isothermal freeze-thaw test was conducted on samples with different clay contents(5%, 10%, 15%, and 20%) at different environmental subfreezing temperatures(-5 ℃, -10 ℃, and -15 ℃), combined with Scanning Electron Microscopy and X-ray Computed Tomography Scanning technology for direct observation and three-dimensional(3D)reconstruction of the cracks, respectively. The development rules and evolution mechanisms of the cracks were investigated. The results showed that the volumetric content of cracks in the samples with 10% and 15% clay content at ambient temperatures below -10 ℃ was relatively high. Cracks propagated from the surface to the inside of the sample. The azimuth angles of the cracks were evenly distributed. The polar angles demonstrated that the cracks were mainly distributed within an angle of ±30° from the horizontal plane. The 3D fractal dimension can well reflect the degree of crack development. The higher the value, the higher the crack content and irregularity. The mechanism of crack evolution can be concluded to be that the clay content affected the initial pore structure of the soil, the morphology of the clay aggregates, and the occurrence characteristics of inter-and intra-aggregate pore water. At different subfreezing temperatures, the difference in ice crystallization pressure inside and outside of clay aggregates leads to morphological changes of clay aggregates, such as shrinkage, expansion, crushing, and rearrangement, and the change of cementation mode between soil particles, which collectively governs the complex crack evolution during freeze-thaw.