Abstract: The purplish-red mudstone of the Badong group of Middle Tertiary, which is widely distributed in the Badong, Wushan and Fengjie County of the Three Gorges reservoir, is often called as sliding-proned formation since most landslides occur in this formation. This paper assesses the expansive potential of the mudstone, weathering conditions, water-rock interaction and other behavior. The results show that the mineral content is approximately 17.9%, of which the montolian constitutes 1.27～6.01%, and the mix layer mineral is over 50%, and it is of a medium to strong gluing capacity and a weak expansive potential. The physical or mechanical weathering is dominant with limited changes in chemical components along the cross-section observed. The water is a dominant factor in controlling the mechnical weathering of mudstone. Therefore, the impacts of water level changes with the reservoir storage and discharge are assessed by field investigation and laboratory tests to assure the stability of the bank slope in the Three Gorges Reservoir.