Abstract: Based on its deformation characteristics under the flexible foundation, the composite ground is divided into four parts, including fill, cushion, composite foundation and underlying layer. Through the analysis of a typical element, a simplified analytical model, which considers the above four parts as an interactive system and allows for the relative movement between piles and soils and assumes different settlements of foundation soil at the same level plane, is presented. And then according to the compatibility of stress and deformation on the interfaces between these four components, the formulas for the settlement and the stress ratio of pile-soil, which is used to characterize the behaviors of the composite ground, are derived. Finally, it is demonstrated that the results from this solution are in accordance with the measured ones of a project. So the proposed method can reflect the working behaviors of the composite ground under the flexible foundation favorably .