Abstract: Crag is one of the major geologic disaster in the Three Gorges Reservoir Region, forming serious threaten to human life and property and to the navigation safety of the river way of the Three Gorges Reservoir Region. As the slide type crag, accounting for 60 %-70 % of the total crags, it is of significant meaning to get clear the failure mechanism of the slide type crag for scientific forecast, prophylaxis and treatment in the Three Gorges limestone area. In this study, it is found that under the control of the geologic structure of paralleled anticline ridge and syncline valley in east Sichuan, the crag is quite developed in the horizontal interbed of limestone and shale or mudstone at the Hengshixi anticline ridge bank slope.The X-Ray diffraction analysis on the soft foundation mineral of the crag rock indicates that the chlorite, mica and montmorillonite have high layered silicates minerals, low shear strength, thus is easy to slide along the potential failure surface in the direction of the major structural surface and leading to the whole rock body sliding down. According to the failure mechanism of the failure process is divided into 4 phases in the paper, i.e. the phases sliding fissure emerge, tensile crack expand, unstable rock sliding and disintegration of rock body. Furthermore, the stress expression of two groups of structural plane that forming the slide is derived in the paper and the mechanical model for analyzing the crag’s stability is established. This method is applied in the analyses on the stability in Wangxia unstable sliding rock, and the results show that the stability coefficient is 1.19 under natural conditions, which belongs to the basic stable rock mass. While, the stability coefficient decreases subsequently as the water head in fissures increases under saturated condition. The research result is helpful to improve our knowledge on stability calculation of the unstable rocks with soft foundations in the Three Gorges area.