Active tectonic activity and intense internal and external dynamic effects,together with abnormal climate change,result in occurrence of many large landslides in the Tibetan plateau.With the newly discovered Lagangcun landslide near the Yarlung Zangbo fault as an example,this study analyzed the characteristics and formation mechanism using field investigation,geological dating,exploratory trench and engineering geological analysis.The results are listed as follow.(1)The Lagangcun landslide is a giant rocky landslide with a total volume of 3.6*10~7 m~3,the maximum sliding distance of 3050m,and the elevation difference of approximately 965 m between the back scarp and the accumulation zone at the leading edge,and the maximum speed of 78.1m/s,indicating that the rocky landslide is characterized with long runout distance and high speed.(2)Influenced by freezing weathering and wedge splitting effect from ice,cracks occur in the rock at the back of the landslide.The glaciers had receded since the Holocene,and melting snow and rainfall infiltration promoted the deterioration of the rock structure,reducing the strength of rock.(3)~(14)C and ~(10)Be age dating shows that the Lagangcun landslide occurred at about 4140~ 9675aBP.The strong earthquake caused by the Yarlung Zangbo fault may be a direct trigger factor of the landslide.The rock mass in landslide area was affected by the throwing force of earthquake,resulting in tensile-shear failure and rapid cut-through of original joint fissures and new fracture surfaces,with failing firstly in broken rock mass near,followed by the upper rock mass losing stability and sliding rapidly downslope.This study can provide references for the research on formation mechanism of giant ancient landslides in the fault zone of the Tibetan plateau.