The special chemical and mineral compositions of marly stones of the Triassic Badong Formation (T 2b) and the moist-hot subtropic environment have resulted in the complexity of engineering geological environment of the resettlement site in the Three Gorges Reservoir region. With a quantitative research on the chemical and clay compositions of marly stones, their weathering rocks and residuals, as well as SEM observations, the process of dissolution of CaCO 3 and concentration of clay minerals with complex components has been disclosed, and the conversion of illite (I) to mixed-layer illite/montmorillonite (I/S) discovered, which explain the reason for deterioration of geotechnical properties of the rocks and soils, thereby enhancing the level of geological and geohazards researches.