Abstract: Landslides are the main geological hazards in Zhangmu port in Tibet.Researching on Zhangmu port ancient landslide is one of the most important factors relevant to the sustainable development of Zhangmu port.On the basis of geologic analysis,authors realized dimensions,nature and mechanism of the ancient landslide trough the whole reconnaissance of the geological hazard again.It was discovered that Zhangmu port doesn’t located on one ancient landslide,but two ancient landslides.The rainfall and eluvial silty & silt near the top surface of bedrock are the two major factors of the ancient landslide formation.The ancient landslide is a debris landslide which was evolved from the rock collapse,whose deformation pattern included bend,tension and crush.The failure mechanism of the ancient landslide included two steps: glide and tensile failure.