Abstract: A practical method for calculating permanent deformation of subgrade soils in flexible pavements is proposed. Based on the general principle that stress analysis and strain calculation are not integrated through the rigorous constitutive matrix in the design and analysis of flexible pavements, a permanent defor- mation model is used in the investigation, and the steps to calculate the permanent deformation are given, with a calculation example demonstrated. Meanwhile, the influence of permanent deformation on the two major pavement distresses is also explored. The general concept proposed in the paper is also helpful to the better understanding of the design theory of highway flexible pavements and railroad tracks.