To mitigate the threat of permafrost slope hazards to transportation engineering in high-altitude regions, and reveal the slope instability mechanisms under freeze-thaw cycles, six covered slopes in western Sichuan are taken as research objects. Based on the hydrothermal coupling theory, freeze-thaw cycle simulation considering diurnal temperature variation in winter is carried out for the six slopes using COMSOL Multiphysics numerical simulation software, and the spatio-temporal variation of hydrothermal field is analyzed. The failure modes of slope are summarized, and the positional relationship between the freeze-thaw interface and the sliding surface is found. The results show that the freeze-thaw cycles mainly affect the soil in the shallow layer of frozen soil slopes. After the slopes are subjected to several freeze-thaw cycles, the moisture content of the soil in a certain depth range is higher than those in other depths, and a water-gathering zone is formed in the slope. Five kinds of slope failure modes are identified from the six covered slopes, which are slope shallow melt soil collapse, slope shallow melt soil creep, slope shallow melt soil extrusion, convex slope shallow melt soil collapse and concave slope shallow melt soil collapse. Under freeze-thaw cycles, the freeze-thaw interface of covered slope is located deeper than the sliding surface.