<p>Karez is a vital groundwater irrigation engineering system in the Xinjiang region. The loess within its underground channels is susceptible to collapse under the influence of seasonal freeze–thaw cycles. To thoroughly investigate the macroscopic mechanical characteristics and mesoscopic failure mechanisms of the deterioration and damage of the loess in Karez underground channels subjected to freeze–thaw cycles, a numerical freeze–thaw model of Karez loess is established using the discrete element method (DEM), and triaxial shear numerical simulation tests are carried out, thereby revealing the freeze–thaw damage mechanism of the loess at both macroscopic and mesoscopic scales. The results show that: (1) Numerical model: A water particle expansion method that accounts for variations in water content is proposed to establish a numerical model. This model simulates the water–ice phase transition and water replenishment process occurring in the loess during freeze–thaw action, thereby reproducing the damage evolution mechanism of its structure under freeze–thaw cycles. A method is developed to characterize water content by taking the volume of water particles as an intermediate variable, enabling the quantitative representation of water content. Verified through comparison with laboratory test results (R² &gt; 0.9), the numerical simulation method and the model are demonstrated to be reasonable and reliable. (2) Macroscopic laws: Under the action of freeze–thaw cycles, the stress–strain curve of the loess descends, whereas the volumetric strain curve rises. The development of freeze–thaw damage in the loess is stage-wise in character, with the most rapid damage occurring during the first five cycles and subsequently tending to stabilize. The failure strength, cohesion, and internal friction angle of the soil continuously attenuate with an increasing number of freeze–thaw cycles, and the rate of attenuation gradually decreases. Confining pressure enhances soil strength through its constraint effect, while an increase in water content significantly weakens the mechanical properties of the loess, with the most pronounced strength deterioration being observed under high water content conditions of 20% and 25%. (3) Mesoscopic phenomena: The freeze–thaw action induces an outward diffusive displacement of the loess particles, with greater displacement occurring in particles near the surface, and the damage and failure progressively propagate from the surface to the interior of the specimen. The cracks generated by freeze–thaw cycles are predominantly tensile cracks, and they are mainly produced within the first five freeze–thaw cycles. The freeze–thaw cycles destroy the bonds between particles, leading to a reduction in the number of medium- and high-strength force chains among particles during shear and thereby diminishing the force transmission capacity of the particles. Conversely, confining pressure densifies the force chain network and constrains particle displacement, thus enhancing the shear strength.</p> Graphical Abstract <p></p>