Abstract: There exists a plenty of ancient landslides in the new Badong county. Examples are Huangtupo landslide and Zhaoshuling landslide. It is of very importance for landslide prevention to study rock mass structure. Firstly, based on geological prospecting data of monitoring profile (300 m deep) at New Badong City, the third member of the Badong Formation of Middle Trassic (T2b3) strata are divided into 24 sections according to the rock core color, lithology and structure. Secondly, according to the tine section identification, x-ray diffraction mineral analysis and detailed measurement of rock core, it is found that the T2b3 strata were deposited in coastal environment that underwent repetitious marine transgression and regression. Lastly, the action mechanism of the interlayer weakness zones with ancient landslides is investigated. It indicates that the 13 strips transfixion weak-soft zone which develops in the T2b3 strata can be divided into a weak intercalation of rich in clayish fine, a cataclasite weak-soft zone, a corrosion transformation weak-soft zone and a soft rock weak-soft zone. Weakness zones are mainly distributed in the open platform (shelf lagoon) and the restricted platform (inter-tidal belt). The interface between T2b3 and T2b3 is continues transition without sliding conditions. The weakness zones give important effect to ancient landslides and will aggravate deep deformation of bank slope under long-term soak when the water level is uplifted to 175 m in the Three Gorges Reservoir.