<p>The equilibrium between the supply and demand of water purification services is essential for safeguarding watershed ecosystems and supporting sustainable socio-economic growth. While current research mainly emphasizes the static analysis of this balance, dynamic investigations into service flow characteristics remain scarce. This study focuses on the Xiangjiang River Basin in southern China, utilizing the InVEST model, differential equations governing supply and demand, and social network analysis to investigate the spatio-temporal patterns of water purification service supply and demand. The results reveal that between 2000 and 2020, the supply of water purification services showed a slight upward trend, while demand exhibited a fluctuating downward tendency, and the supply-demand surplus fluctuated. On an annual basis, the supply generally met the demand, but significant spatial variation in supply-demand balance was observed, with high-risk zones concentrated in the central Hengshao arid corridor and the northern Changzhutan metropolitan area. In the water purification service network, surplus nodes increased, while deficit nodes decreased over time. Network density improved from 98.96% in 2000 to 100% in 2020. The flow of surplus services from upstream regions alleviated supply-demand imbalances downstream, notably in the Hengyang-Shaoyang (Hengshao) arid corridor and the Changsha-Zhu-zhou-Xiangtan (Changzhutan) metropolitan area. However, during drought years, diminished water volume intensified supply-demand pressures in these regions. This research framework, which incorporates the dynamic effects of service flow, broadens the scope of water purification service studies and provides a solid foundation for integrated water resource and environmental management.</p>