The main dam of the Datengxia hydro-project is a concrete gravity dam. According to survey and excavations, many soft and weak interlayers develop in the dam foundation rockmass of the release sluice segment. The soft and weak interlayers are characterized by poor performance, good continuity, gentle dip towards the downstream left bank, forming a deep sliding boundary of the dam foundation. In this study, the mechanical parameters of structural plane are determined by analyzing the distribution and properties of these soft interlayers, and the slid patterns are determined by the spatial combination of soft interlayers and faults. Moreover, the sensibility of sliding surface to depth is analyzed using rigid limit equilibrium method, and the deep anti-sliding stability of dam foundation is calculated and evaluated. According to the calculation results, treatment measures are put forward for dam segment which do not meet requirements. This research provides a typical case for the study of deep anti-sliding stability of gravity dams in large hydro-projects, and also provides enlightenment and reference for similar projects around the world.