Due to the complexity of soil-structure interaction, simple constitutive models typically used for interface elements in general computer programs cannot satisfy the requirements of discontinuous deformation analysis of structures that contain different interfaces. In order to simulate the strain-softening characteristics of interfaces, a nonlinear strain-softening interface constitutive model was incorporated into fast Lagrange analysis of continua in three dimensions (FLAC3D) through a user-defined program in the FISH environment. A numerical simulation of a direct shear test for geosynthetic interfaces was conducted to verify that the interface model was implemented correctly. Results of the numerical tests show good agreement with the results obtained from theoretical calculations, indicating that the model incorporated into FLAC3D can simulate the nonlinear strain-softening behavior of interfaces involving geosynthetic materials. The results confirmed the validity and reliability of the improved interface model. The procedure and method of implementing an interface constitutive model into a commercial computer program also provide a reference for implementation of a new interface constitutive model in FLAC3D. Due to the complexity of soil-structure interaction, simple constitutive models typically used for interface elements in general computer programs can not satisfy the requirements of discontinuous deformation analysis of structures that contain different interfaces. In order to simulate the strain-softening characteristics of interfaces, a nonlinear strain-softening interface constitutive model was incorporated into fast Lagrange analysis of continua in three dimensions (FLAC3D) through a user-defined program in the FISH environment. A numerical simulation of a direct shear test for geosynthetic interfaces was conducted to verify that the interface model was implemented correctly. Results of the numerical tests show good agreement with the results obtained from theoretical calculations, indicating that the model incorporated into FLAC3D can simulate the nonlinear strain-softening behavior of interfaces involving geosynthetic materials. The results confirmed the validity and reliability of the improved interface model. The procedure and method of implementing an interface constitutive model into a commercial computer program also provide a reference for implementation of a new interface constitutive model in FLAC3D.