Abstract: Based on the generalized plasticity theory and critical state concept, a three-dimensional constitutive model for unified modelling of sand behavior is proposed with a set of material parameters. By the User-defined Material (UMAT) subroutine interface in ABAQUS, the constitutive model of sand is developed and compiled with Fortran. The comparison between the simulations and experimental results of three kinds of sand, i.e., the Toyoura sand, the Fuji River sand and the Tokachi sand, shows that the finite element method could effectively simulate the stress-strain curve over a large range of confining pressures and initial densities. Therefore, the model can better reflect the deformation and strength characteristics of soil under a three-dimensional stress state, where the phenomenon that loose sand contracts while dense sand dilates under shearing is well described. The research can be further applied to numerical analyses related to geotechnical engineering to provide more efficient and practical solutions.