In the Western Sichuan area, settlement or deformation problems often occur in embankments consisting of the mixed soil containing sandy gravel and clay particles. The accumulated deformation under traffic loads could be described by the dynamic constitutive model under cyclic loading. The proposed model uses the mapping rule of mobile mapping origin and integrates the loading-collapse yield criterion within the framework of elastic-plastic bounding surface model. Modified Cambridge model was introduced to define plastic potential equation. Saturation and fine particle content are considered as two main influencing factors. The model parameters are obtained by fitting measured results. Comparisons between experimental results and model simulations show that the elaborated model is capable of describing the unsaturated loess behaviors under static loading and cyclic loading, and predicting the hysteretic properties during both loading and unloading. This model provides theoretical basis for settlement prediction of mixed soil subgrade in the Western Sichuan area.