Abstract: This paper presents an improved three-dimensional anisotropic elastic-viscoplastic constitutive model based on overstress theory of Perzyna. A rotational hardening law of Wheeler is adopted to account for the initial anisotropy and the changes in anisotropy due to the viscous strains. The developed model is implemented by using ABAQUS software with UMAT subroutine, which is integrated with a return mapping algorithm. The undrained triaxial test for HKMD under complicated loading conditions and the multi-stage undrained triaxial creep test for Sachville clay were simulated by the proposed model. The simulated results show that the proposed anisotropic elastic-viscoplastic model can successfully reproduce the rate-sensitivity, relaxtion, creep and anisotropic behaviors of soft clays under different loading conditions.