[Purposes] This paper aims to develop and validate a constitutive model of interfacial soil considering the influence of microstructure. [Methods] A microstructure scalar considering the influence of the directional arrangement of particles and particle fragmentation was introduced into the model, which affected the distance between the shear expansion state line and the critical state line. The microstructure-dependent fractional-order constitutive model was built by using the fractional-order plastic constitutive method of soil. The Euler-Cauchy algorithm with error control was used to compile the established model in Matlab and verify the matching between the numerical modeling and the stress-deformation law of the actual test data under three-dimensional and two-dimensional conditions, different boundary conditions, and different microstructure boundary values, respectively. [Findings] The fractional-order constitutive model of interfacial soil with 10 model parameters is obtained, and the model can accurately describe the three-dimensional shear behavior of interfacial soil under the condition of three-dimensional constant normal load (CNL)/stiffness (CNS). The microstructure scalar evolves towards its boundary value, and the predicted values of the model are higher than the measured values under different microstructure boundary values, but the stress displacement variation of the predicted values is consistent with that of the test data. [Conclusions] The established model can well simulate the mechanical behaviors of interface soil such as hardening, softening, shear expansion, and shear shrinkage. All the model parameters can be obtained by test, which can provide a reference for the constitutive theoretical research on the soil-structure interface considering the influence of microstructure.