Abstract: Soft soil exhibits typical nonlinear rheological characteristics, which cannot be accurately described by traditional element models. By replacing the traditional damper with fractal damper and considering the damage effect of soil, an improved nonlinear fractal derivative Nishihara viscoelastic-plastic rheological model which can fully describe the decay stage, steady stage and accelerated stage of rheology is proposed and the analytical solution is given. The fitting result of the proposed model is in good agreement with the test result, which proves its higher precision and simpler calculation. The corresponding UMAT subroutine of the improved rheological model is compiled by ABAQUS, and the effectiveness is verified by triaxial creep tests and an embankment case. The comparison results show that the established model and developed subroutine can accurately reflect the complete three-stage viscoelastic-plastic rheological characteristics of soft soil. The research results provide reference for the establishment of viscoelastic-plastic fractal derivative rheological model in soft soil and its secondary development and application in ABAQUS.