The properties of cumulative deformation of  red mudstone soils, a special grain material, under cyclic loading are investigated through dynamic triaxial tests. When dynamic load is below the critical dynamic stress, the variation of cumulative strain of red-mudstone soils has its own laws: cumulative strain grows rapidly in the early stage; with the increase of load cycles, the increasing rate of strain slows down, and the strain finally reaches stable. At the same time, most of the deformation occurs before the first 1 000 cycles. On the basis of dynamic triaxial test results and in consideration of main factors such as soil type, soil’s stress state, soil's physical state and the number of load cycles, a model to calculate the cumulative strain of red-mudstone soils subjected to cyclic loading is put forward. This model can reflect the influences of dynamic deviatoric stress, static deviatoric stress, static strength and confining pressure. Meanwhile, the model parameters have clear physical meaning and are easy to determine. The validity and practicality of this model is verified through field excitation tests on red mudstone subgrade of Dazhou-Chengdu Highway.