Understanding the formation and distinctive conditions that contribute to icing in cut slopes are needed to mitigate it for highway engineering. Using the K162 cut slope of the Bei’an-Heihe Expressway as a study site, we conducted field surveys, geological exploration, field monitoring, laboratory tests and numerical simulations to carry out an integrated study on the icing formation mechanisms and its influence on the slope stability. Research results show that: the surface unconsolidated Quaternary sediment and Tertiary sandstone provide passage for atmospheric precipitation infiltration; but underlying mudstone forms an aquiclude. Phreatic water forms in the loose overburden after infiltration. As the freezing front thickens, the phreatic aquifer thins and becomes pressurized. Slope cutting has exposed the phreatic aquifer. When the excess pore water pressure exceeds the strength of surface material, the pressurized water flows out of the slope, and freezes, forms icing. In the spring melt period, surface icing and shallow seasonal frozen soil melt completely, water infiltrates into the slope; but meltwater is blocked by the unfrozen soil in infiltrating process, accumulates on the interface between melted and frozen layers, increasing the water content at the mudstone interface. The mudstone reaches a saturated state, and its shear strength decreases, and forms a potential rupture surface.