As the water source of the Middle Route of China’s South-to-North Water Transfer project, the change of water quality of Danjiangkou Reservoir has been attracting much attention. This study aimed to obtain the annual water quality pattern and influencing factors of Danjiangkou Reservoir by means of multivariate analysis and correlation analysis using monthly data surveyed in 2021. The results of PCA and RDA variation partition analysis showed that the water quality of Danjiangkou Reservoir was mainly affected by temporal factors, and the spatial effect was limited. The trajectory analysis results revealed that the unusual floods in the current years have specific impacts on water quality in Danjiangkou Reservoir at the temporal and spatial scales. The results of Pearson and Mantel correlation test indicated that the inflow and water level affected the spatial-temporal pattern of reservoir water quality mainly via the NO₃^– and PO₄³⁺. With the global change i.e. extreme flood, this research would provide a basis for the decision-making of management empirically.