Abstract: Knowledge about the dissolution process of the marlite karst is a key to understanding the engineering characteristics and genesis of weathering rocks and their residual-to-diluvial-type expansive soil. However, this line of study has not received enough attentions in karstology research. Through analysis of SEM photos of samples of marlite with different weathering degrees from the Triassic Badong formation(T 2b) in the Three Gorges Reservoir area, the authors found that the dissolution within marlite results in the variation of its micro-structures. Such variation could in turn reduce the mechanical strength of marlite when the dissolution of micro-holes occurs. Furthermore, geo-disasters such as differential subsidence, fissures, landslide, collapse and cave-in could result from the development of large dissolution holes or deformation of residual-to-diluvial-type expansive soils.