In many parts of the world, water retained in snowpack and glaciers are critical to the stability of downstream ecosystems and human society. This study sets out to quantify, map, and explain the spatiotemporal pattern of annual snow cover duration, which are quantified by regional differences in the number of annual snow cover days (Dsc) and its sensitivities to climate change across the Qilian Mountains between 2001 and 2018. The Qilian Mountains, based on the previous studies on regional climate difference, can be divided into three subregions from east to west. Across the entire Qilian Mountains, Dsc is about 20-40 days at elevations below 3500 m.a.s.l and increases to about 130 days at 4500 m.a.s.l, and more than 300 days above 5000 m.a.s.l. In the west, the interannual average Dsc is consistently lower than the east at the same elevation, and the largest difference is about 200 days at 4300 m.a.s.l. The middle subregion has its Dsc lower than the west below 4000 m.a.s.l, but higher above that elevation. Throughout the 18 years, a 100-200 m upward shift of the RSLE in the hottest months (July and August) has happened in all the subregions of the Qilian Mountains, which certainly results in a decrease of snow cover area and duration. The statistical analysis indicates that on the 3500-4500 m.a.s.l hillside regions in the west section, the number of annual snow cover has the most pronounced decline of Dsc with a rate from -3 to -5 d/year, and this declining trend is caused by the 0.1-0.2 ℃/year increasing trend of the local land surface temperature. The sensitivities analysis of Dsc to temperature and precipitation suggests that, in all the subregions, the annual mean temperatures T has a consistent first-order control on Dsc across the years. However, throughout the elevation, neither the temperature nor cold half-year precipitation has consistent first-order control on Dsc. On the contrary, the data highlight the importance of the interaction between the two climatic variables at 3500-5000 m.a.s.l, where Dsc is sensitive to local climate change. In this elevation range, the positive effects of cold half-year precipitation increasing may partly compensate for the negative effects of temperature rising on Dsc. Furthermore, since the lack of precipitation in the west section, the interplay between temperature and precipitation has its influence on Dsc mostly in the east section, and as a consequence, the significant reduction of Dsc in a large area only happens in the west of the Qilian Mountains. Moreover, we look into the correlation between Dsc and the hillshade and get an interesting result that the medium-hillshade regions, but not the lightest regions, always have the lowest Dsc. The possible reason for this phenomenon is that the lightest regions are more likely the peaks of the mountains i.e. glaciers, while the medium-hillshade regions are more likely the hillshade at low or medium elevation.