Catastrophic failure of slopes often results from cyclic softening, i.e. onset of significant strains or strength loss in soils during earthquakes. Therefore, it is necessary to study the nonlinear dynamic constitutive model considering cyclic softening in order to analyze the stability of seismic slope under complex conditions. On the basis of existing nonlinear dynamic constitutive models, a method to deal with the cyclic softening is proposed. A secondary development of the constitutive model is implemented into the FLAC3D platform, and the proposed procedure is verified by the theoretical formulation and the experimental data available in the references. It is shown that the calculated backbone curve is consistent with the theoretical formula, and the calculated dynamic shear modulus ratio and damping ratio are in good agreement with the experimental data. When the shear strain is larger than >0.01%, the modified model describes better the damping ratio-shear strain relationship than Hardin-Drnevich model and Davidenkov model. If the cyclic softening is taken into account, the calculated shear strength is significantly reduced, and the convergence of the model is improved when the shear stress of the backbone curve can be continuously transferred to the softening main curve. The developed constitutive model can provide support for the seismic disaster assessment of soft soil site and slope under large strain condition.