Ecosystem service function refers to the natural environmental conditions and utilities that the ecosystem forms and maintains to promote human survival and development. Many ecosystem service functions are of great importance to human wellbeing, especially those related to water. Water yield is a key index characterizing the regulation function of ecosystem in watershed and plays a critical role in the sustainable development of these sectors, including agriculture, industry, fisheries, and domestic activities, as well as directly impacts the regional economy and society. Climate and land use changes are the main factors affecting the spatiotemporal variation of water yield. In the context of global climate change, the quantitative assessment of water yield makes significant sense on formulating the water resource protection planning, establishing the ecological compensation mechanism and maintaining the ecological balance in the watershed. However, little research has been conducted on the spatiotemporal variation of water yield and its influencing factors in the alpine region of western China. The Shule River Basin, located in the northeastern margin of the Qinghai-Tibet Plateau and the western part of the Qilian Mountain, is the "Lifeline" and "Natural Water Tower" of farmers and herdsmen in the Hexi Corridor region of Northwestern China. This study, using the upstream regions of the Shule River Basin as the study area, evaluated the spatiotemporal variation of water yield in 2001-2019 based on Integrate Valuation of Ecosystem Services and Tradeoffs Tools (InVEST) model and examined the relation between the water yield and influential environmental factors using correlation analysis. To evaluate the performance of the InVEST model, we analyzed the relationship between the simulated total water yield and measured total water yield. It showed that the InVEST model had a good performance in estimating water yield (R²=0.986, P<0.05) for alpine region. The results demonstrated that: (1) the mean annual temperature and annual actual evapotranspiration showed an increasing trend (P>0.05), while annual precipitation and Normalized Difference Vegetation Index (NDVI) increased significantly (P<0.05) in 2001-2019. (2) The mean annual total water yield was 13.66×10⁸ m³, and the water yield was higher at the mountainous high-altitude regions and lower at the low-altitude regions of river valley. (3) The water yield significantly increased with a rate of 0.26×10⁸ m³/a from 2001 to 2019 (P<0.05), and the water yield significantly increased in the high-altitude regions (accounting for 32.01%), while the low altitude regions of the northwest significantly decreased and accounted for only 8.39% (P<0.05). (4) There were significantly positive correlations between water yield and precipitation, and negative correlations between water yield and air temperature, actual evapotranspiration, NDVI and root depth (P<0.001). (5) The water yield function of the generally important area which refers to poor water yield function had a proportion of 47.26%, whereas the very important region accounted for 28.25% of the total area. The results can provide scientific insights for dynamic assessment, effective management, and sustainable development of water resources in the Qilian Mountain National Park of China.