Lying between 80.33°-94.93°E and 28.76°-36.49°N, the alpine pastoral area of northwestern Tibet is known as the ridge of the roof of world with a mean elevation of 4500 m. As an alpine pastoral area, the animal husbandry faces the dual pressures of both gaining the yield and protecting the pasture. The environmental condition of this area has significant effect on the main rivers, climate and eco-environment of Tibet and the whole country, even on the globe for its rigorous natural condition and fragile ecosystem.This study selected vegetation cover as the main evaluation index, and analyzed the present situation of grassland degradation and it’s mechanism of the alpine pastoral area in northwestern Tibet, based on the remote sensing data such as NDVI data derived from NOAA/AVHRR with a spatial resolution of 1 km of 1992-2005 and from MODIS with a spatial resolution of 0.5 km of 2002-2005 respectively in this area, in combination with investigation data of the actual condition of grassland resources completed according to the background material of the end of the 1980’s. The grassland degradation processes and their response to climate change during 1971-2005 were discussed and analyzed systematically in the study using meteorological data.The results show that grassland coverage rank degree presents normal distribution with ground vegetation being sparse as a whole in this region. According to the present degenerated situation, the total degenerated grassland area is 14.19×10⁴km² , occupying 39.64% of the region’s total natural pasture area. Of which light degenerated area is most extensive, being 65.96%, moderate and heavy degenerated areas are 25.20% and 8.84% respectively of the total degenerated grassland area.From the analysis, we can conclude that the direct factors related to grassland degeneration in the study area are climate change for the annual temperature increasing with cess precipitation in recent years in Naqu county. The indirect factors are over-grazing and human activities. For example, the overload rate reaches 59.18% as a result of domestic animal’s massive increase, thus bringing increasing pressure on part of pasture, and causing grassland degeneration and more and more serious desertification phenomenon. In addition, population increase and frequent human activities also bring serious damage to the grassland.