[Objective] Accelerating urbanization has deteriorated the balance between water storage and discharge，leading to frequent extreme flood disasters in river basins. Currently，there is a lack of quantitative indicators to characterize the storage-discharge relationship in highly urbanized watersheds，and the underlying mechanisms driving its spatiotemporal evolution remain inadequately understood. This study analyzes the evolving characteristics and influencing mechanisms of the storage-discharge relationship in highly urbanized watersheds，aiming to reveal how extreme rainfall-runoff events impact this fundamental hydrological relationship. [Methods] Taking the Qinhuai River Basin as the study area，we constructed storage-discharge relationship curves and selected the curve slope as a characteristic indicator to quantitatively characterize the relationship and its spatiotemporal evolution. Furthermore，Granger causality tests and partial correlation analyses were employed to identify the influencing factors of the storage-discharge relationship and to reveal its response characteristics under extreme storm scenarios. [Results] （1） The dominant function of the Qinhuai River Basin shifted from water storage to drainage. While storage dominated from 1990 to 1999，the basin transitioned towards drainage after 2000，although dynamic storage volumes remained above 100 mm. Post-2010，intensified urbanization made the drainage-dominant pattern more pronounced. （2） The nonlinearity of the storage-discharge curve exhibited a “weakening-then-strengthening” trend，with urbanization and extreme storms significantly amplifying this nonlinearity. Hydrological responses differed between low- and high-flow regimes，with the latter likely influenced by anthropogenic regulations. （3） The influencing factors of the storage-discharge relationship varied dynamically with flow levels. At low flows，the relationship was primarily driven by extreme precipitation and drainage capacity. At medium flows，the midstream and upstream areas showed a pronounced response to extreme storm characteristics. At high flows，despite increased frequency and intensity of extreme storms，the interaction between the storage-discharge relationship and extreme storm indices weakened，necessitating anthropogenic regulation to ensure basin safety. （4） Urbanization significantly impacted the storage-discharge relationship，generally reducing basin sensitivity and weakening regulation capacity. The upstream Lishui River exhibited the highest sensitivity （0.005，0.013，and 0.03 mm^-1 under high，medium，and low flows，respectively） and the largest variation amplitude under extreme storms，marking it as a critical area for focused management. [Conclusion] The quantitative indicators and methodologies proposed in this study for analyzing storage-discharge relationships provide a valuable scientific reference for urban flood control planning and the sustainable development of river basins.