[Objective] Gannan Prefecture, Gansu Province, is an important water source conservation and supply area in the upper reaches of the Yangtze River and Yellow River, and it is also a crucial ecological security barrier of China. The spatial distribution pattern, temporal trend, and driving factors of water source conservation in Gannan Prefecture were studied to provide references and bases for regional water resource management and for promoting the development of healthy ecosystems. [Methods] The spatiotemporal characteristics of water source conservation in Gannan Prefecture from 2005 to 2020 were quantitatively analyzed based on the InVEST model, and an analysis of its driving factors was conducted using the geographic detector model. [Results] ① The average annual water source conservation depth and total conservation volume in Gannan Prefecture were 25.77 mm and 9.45×10⁸ m³, respectively. The overall spatial pattern showed higher values in the southern part and lower values in the northern part of the study area. Grassland and forestland dominated the water source conservation function, accounting for 65.92% and 29.43%, respectively, of the total water conservation capacity. ② Over the 16-year study period, water source conservation showed an overall increasing trend, with an average growth rate of 1.10 mm/yr. In terms of administrative divisions, areas with high water source conservation values were mainly located in the southern part of Maqu, the southern part of Diebu, and Zhouqu. In terms of topographic factors, high values were mainly concentrated in the elevation range of 3 000—4 000 m and in the slope range of 8°—25°. ③ The geographic detector analysis found that precipitation was the main factor affecting the temporal variation of water source conservation in the study area, while precipitation and land use were the dominant factors influencing the spatial distribution of water source conservation. [Conclusion] Water source conservation in Gannan Prefecture exhibited obvious temporal evolution laws and spatial distribution patterns. The dual influence of natural factors (mainly precipitation) and human activities (mainly land use) was the main driving force affecting spatiotemporal changes in water source conservation.