The China-Nepal Highway, located in southwest Tibet, is vital not only to the economical development of Tibet, but also to national defense in China. Because of the complex of geologic structures, complicated stratigraphic and lithologic characters, intensive neotectonic movements, a great variety of hydrothermal conditions and strong glacial actions, all kinds of natural hazards occur widely on both sides of the highway, such as debris flows, landslides, avalanches, etc. The above hazards often threaten the transportation safety of the highway, and severely restrict local economic development. Among all the mountain hazards, debris flow is the major one. In this paper, based on historical data, the interpretation of aerial photos and satellite images (TM and SPOT), as well as field investigation, the environmental background of the study area, especially the geological structure, lithology, plant and soil, hydrography and climate are analyzed in detail. Some 842 debris flow gullies are identified in this area, of which 474 have clear channels and 241 develop on both sides of the highway. The mouths of debris flow gullies are located at elevations between 1810 m and 5000 m, of which more than 80% range between 3500 m and 4500 m. The drainage basins of debris flow gullies cover an area between 0.2-161 km2, especially, 75% of which with an area between 1.0-5.0 km2. According to their genesis, debris flows in this area can be divided into 3 types: glacial, glacial lake burst and rainfall. Of the 241 debris-flow gullies, 205 (85.1%) are of rainfall debris flows and 26 (10.8%) are of glacial debris flows.