Abstract: A plastic accumulative deformation constitutive model of soils under a long term traffic load is very important in the foundation settlement calculation. The existing soil element model can be used to calculate the cumulative strain of the rock and soil body, but without consideration of the cyclic rotation of principal stress axes of the saturated soft clay. For the long-term settlement of subgrade caused by traffic loads, the cyclic principal stress rotation might be important. On the basis of the a series of loading tests with the principal stress rotating under the isotropic consolidation and a series of cyclic torsional tests with anisotropic consolidation, the Newton dashpot is replaced by the Abel dashpot and the fractional order derivative is established. The parameters of the Burgers model and the fractional order derivative Burgers model of the axial plastic cumulative strain are optimized by a genetic algorithm. By analyzing the correlation curve of the calculation value and the test value of the two models, the fractional order derivative Burgers model is found to be more appropriate for calculating the cumulative strain of the soft clay subjected to a cyclic rotation of principal stress axes.