Abstract: There are still no effective methods to predict the settlement of subgrades caused by long-term irregular traffic loads. The mechanism of the plastic deformation caused by slight long-term dynamic loads has not been understand. And the existing plastic theory can hardly involve this type of deformation. In order to solve this problem, a practical method is proposed based on an equivalent vicso-elasto-plastic model. The model consists of two components. The first is a visco-elastic model, in which dynamic modulus is determined by Ramberg-Osgood equations and the damping ratio is determined by the equation proposed by Hardin and Drnevich. The other is empirical equation determines the residual strain, which is based on the stress process in subgrade foundation caused by traffic loads and the characteristics of residual strain tested by dynamic triaxial tests. After the stress process is obtained using the finite element analysis, the residual strain of every element can be determined by the empirical equation, then the settlement of the subgrade can be predicted. A simple example is presented to illustrate this procedure.