Abstract: The severity of grassland degradation was assessed between two periods of 1990s and 2008 for the sources region of the Yangtze River, the Yellow River, and the Lancang River in west China, which includes counties of Chenduo, Maduo, Yushu, and part of Qumalai, Zhiduo, Zaduo, Nangqian, and Dari. TM images, spectral reflectance and the percentage of grassland coverage obtained by field surveying, as well as GPS technology are employed. Grassland coverage, which is one of the most important indicators for evaluating grassland vegetation dynamic change, was estimated by a quantitative model. The grassland coverage indicators of two period images were compared with pixel by pixel. The change of grassland vegetation in temporal and spatial was discovered. The paper compared the correlation between NDVI and grassland coverage, which were both surveyed by samples, and the correlation between NDVI of TM image and the coverage of grassland samples. And the paper calibrated TM image NDVI by model of NDVI and grassland coverage surveyed by samples. The quantitative model was made by calibrated TM image NDVI and surveyed grassland coverage. The correlation between the coverage of grassland samples surveyed in 2008 and the one estimated by TM image was 0.84, and average assessing accuracy was 80.18%. The assessing accuracy was 77.46%, by comparing results of grassland coverage distribution between methods of the paper and spectral mixture analysis. Method of the paper is verified effectively by analyzing and comparing results of the paper and grassland change regularity of nature reserves .The result indicates that part of the research region grassland is degraded, while part is recovered. 0.16% of the study area is highly degraded, 11.40% is moderate degraded, and 31.89% is slight degraded, while 56.55% of the region grassland is recovered in some degree. The regions of recovered and slight degraded are located in nature reserves of the sources region of the Yangtze River, the Yellow River, and the Lancang River. It indicates that country nature reserve construction has taken effective measures to solve grassland degradation problem.