Abstract: The stress of a soil particle at a point in the soil-structure interface layer not only depends on the strain and deformation history of the point，but also depends on the stresses of other points near the point． According to the early non-local theory，considering the influence and interaction between the tiny structures of the non-homogeneous materials，the non-local plastic shear strain was expressed as the weighted average of the corresponding parameters of itself，and also considering the effect of rough ness of structural surface on the mechanical characteristics of the soil in the interface layer，the constitutive model of soil-structure interface based on strain gradient plasticity theory was established under relatively low normal stress condition． Each parameter in the model had a clear physical meaning． The results showed that the model could be used to study the relationship between shear stress and shear strain， especially in the case that the characteristics of strain softening after the peak shear stress．