Abstract: Abstract: With the increasing of the number of structures in permafrost regions or structures (such as urban underground and mine shaft engineering) using freezing method, the properties of interface layer between frozen soil and structure are receiving more attention. Under the action of loads, the mechanical responses of interface layer are different from frozen soil and structure material. The interface layer between frozen soil and structure is vulnerable to severe damage under the loads of gravity, construction and earthquake, and thus will affect the safety and durability of structures. The newly developed mechanical testing apparatus is used to test the mechanical characteristics of interface layers between frozen soil and structure. Based on the existing apparatus called large-scale frozen soil direct shear system (DDJ-1) in our laboratory, the shearing box of frozen soil is modified to highlight the interface layer of frozen soil, and the measuring system of tiny deformation is developed, which thus constitute the experimental system. Micro deformation measuring system is composed of digital imaging system (DIS) and digital image processing software system (DIPSS). DIS consists of high definition and resolution camera (JHSM1400) and 7.2 mm distortionless industry fixed-focus camera, and DIPSS has functions of calibration setting, measurement setting and data display. The data received from the system are accurate and the error is about only 1 μm. The newly developed mechanical testing apparatus is used to test the mechanical characteristics of interface layers between frozen soil and rough steel plate under the monotonic load. The mechanism of basic forces and deformation of the interface layers is analyzed from the perspectives of macro mechanics and micro deformation. The results show that the peak shearing stress, stable shearing stress and initial shearing stiffness increase with normal stress, and shearing strength of the interface layer is correlated with normal stress. The relationship between this maximum shear stress and normal stress follows the Mohr-Coulomb law. The value of friction angle increases with the roughness of interface at each temperature, and the value of friction angle of interface with the same roughness increases with the decrease of temperature. The value of friction angle of interface is close to that of frozen layer, which demonstrates that the shearing failure occurs in the internal of frozen soil. The shearing displacement of frozen soil particles decreases with the increase of interface layer depth, and increases with the increase of normal stress in the same depth of interface layer; and deformation characteristics of interface layer is similar among the structures with different roughness. The thickness of interface layer is about 0.5-3.5 mm within experimental conditions. The deformation of frozen soil can be decomposed into 2 parts, i.e. slide deformation at the interface and shearing deformation under constraint. The shearing strength of interface layer increases with the decrease of frozen soil temperature and when the temperature is below -10℃, the freezing force is raised significantly. Meanwhile, the shearing strength of interface layer increases with the roughness of structures and when the roughness is above 0.8 mm, the increase rate of shearing stress reduces. The roughness of interface, the temperature of frozen soil and the normal stress have profound effect on mechanic properties of interface layer.