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One of the focuses of global change research is on the impact of climate change on alpine vegetation. The Altun Mountain National Nature Reserve is the largest alpine desert rangeland reserve in China to protect wild endangered ungulate species. This paper aims to detect changing trends in rangeland conditions in this region. Temporal changes in the Normalised Difference Vegetation Index (NDVI) for the rangelands in the Altun Nature Reserve and its correlation with climatic variables were studied over the period from 1998 to 2012. Based on the NDVI index and using ArcGIS spatial analyst, the areas of likely rangeland degradation and areas of improved in rangeland condition were identified using linear regression analysis. The results showed that NDVI values were relatively low, varying from 0.04 to 0.1, and there existed distinct monthly changes. The highest NDVI values were exhibited in August. Generally, the NDVI showed an increasing trend over time with several annual fluctuations. High values were distributed mainly in the core area of the nature reserve. Trend analysis showed that vegetation near rivers and lakes was most likely to be degraded but, overall, the vegetation conditions improved over the 15 years of the study, which meant an improvement in the habitats of key wild ungulate species. Precipitation and temperature had a significant linear positive correlation with NDVI, which suggested that they were the main driving forces for rangeland improvement. The vegetation at the edge of the protected areas appeared degraded due to human activities.