The studied area is located in the south marginal belt of the Qingzang Plateau where the tectonic movement is exctensive,the mountain uplifting and river undercutting region are quickly, inner the basin,the g radient of gullies are always 22～35°,the relative difference of height is 400～1 500m,these landscape conditons are advantageous to the formation of debris flow；nearly every extensive tectonic movenent,the quarternary glaciation,subglaciation and surfacial weathering generated many kinds of loose deposits and formed the solid material foundation for formation and development of debris flow；the annual precipitation is about 450mm, among which the rainfall during summer occupies nearly 90 percent,the rainfall during frequently ocurring period of debris flow in July and Augest occupied 60 percent of the annual precipitation,the 10 minutes rainfall intensity is 6. 2mm and 7. 6mm in Yang bajing and Lahsa respectively.The rainfall in studied area has the high intensity features of mountain area, in another word, the regional rainfall is quite favourable to formation and activity of debris flow.