Abstract: To elucidate the evolution law governing the mechanical properties of the remolded soil-undisturbed soil interface in slope cutting and gully filling projects in Northwest China, this study conducted direct shear tests and scanning electron microscopy (SEM) tests on the interface under different freeze-thaw cycle counts, moisture contents, and compaction degrees. The influence mechanisms of various factors on the interface’s shear strength characteristic parameters and microscopic pore structure were elucidated. Additionally, response surface models correlating the interface’s internal friction angle and cohesion with each influencing factor, and linear relationships between the aforementioned shear strength parameters and equivalent pore diameter, were established. Results indicate that at the macro scale, under freeze-thaw cycling, the interface’s peak shear strength, effective cohesion, and effective internal friction angle all exhibit an exponential decline, with cohesion reducing more significantly than the internal friction angle,and there is no significant interaction effect of various factors on the interface’s cohesion and internal friction angle. At the micro scale, during the initial phase of freeze-thaw cycles, pore morphology undergoes significant changes: pores gradually expand,some interconnect to form cracks, and the proportion of large pores increases, though micropores and small pores remain predominant. Furthermore, there is an inherent consistency between soil microstructural evolution and macroscopic mechanical response. Deterioration of the microscopic pore structure gradually weakens interparticle interlocking effect and cementation, which manifests macroscopically as concurrent attenuation of the interface’s mechanical properties. The results of this study can provide experimental evidence for stability evaluation and protection design of the remolded soil–undisturbed soil interface in slope cutting and gully filling projects in cold regions.