To reveal the impact of landscape pattern evolution in the hilly mountainous regions of Southern China on water quality, this study analyzed the spatiotemporal changes in landscape element characteristics and landscape heterogeneity, as well as their response relationships with water quality indicators, using landscape pattern indices and the InVEST model based on water quality monitoring data from the Jiuqu Creek Basin in Wuyi Mountain. The spatiotemporal differentiation of water source conservation and water purification services were also be assessed. The results indicate that the vegetation in the study area is primarily composed of mixed coniferous and broad-leaved forests, evergreen broad-leaved forests, and bamboo forests, with relatively low landscape diversity. The water quality exhibits a pattern of superior conditions in the upstream and deteriorating conditions in the mid-to-lower reaches. From 2010 to 2020, the area of tea gardens expanded, landscape fragmentation increased, and water quality indicators deteriorated. Landscape fragmentation was significantly negatively correlated with fecal coliforms and suspended solids. The simulation results of the InVEST model show that the upstream unit area water conservation capacity increases, and the middle reaches are the core area of nitrogen and phosphorus pollution. It is suggested that ecological monitoring and digital management throughout the entire watershed to protect natural vegetation should be strengthened. Differentiated protection should be implemented for the upstream, midstream, and downstream, with strict ecological red lines in the upstream, control of agricultural non-point source pollution in the midstream, and improvement of urban stormwater management systems in the downstream.