[Objective] Research on the discharge regulation and downstream water-level response mechanism of a super-large hydropower station is of great significance for ensuring downstream navigation safety and ecological protection. This study aims to reveal the hydrological variation mechanism in the reservoir tail reach under ultra-high dam operation and to provide a theoretical basis for coordinated cascade reservoir operation and navigation safety assurance. [Methods] Based on hourly discharge and water level observation data at the downstream section below the dam in 2023, the standardized hourly discharge series was analyzed using Morlet continuous wavelet transform. The main periodic components of the time series were identified using wavelet variance analysis, and the regulation cycle characteristics were extracted and the discharge-water level response relationship was quantified to systematically reveal the spatiotemporal response mechanism between discharge variation and downstream water level variation. [Results] (1) The discharge of the hydropower station under study exhibited two dominant periods of 38 h and 19 h, corresponding respectively to a diurnal regulation cycle and a semidiurnal peak-valley rhythm, and the operational intensity increased significantly during the flood season and storage period. (2) The water level variation at the downstream main control section showed significant spatiotemporal heterogeneity. During the main flood season (July-August), jointly driven by flood control operation and diurnal regulation, the peak daily water level variation reached 10.94 m, and the instantaneous variation reached 3.92 m, which was 6-19 times higher than that at the downstream reference section (permanently backwater-influenced reach). (3) A significant positive correlation was observed between discharge variation and water level variation (R²=0.83). An increase of 1 000 m³/s in discharge corresponded to an average water level rise of about 2 m. A discharge fluctuation of 800 m³/s could induce an instantaneous water level variation of [-1,1]m. (4) A rapid attenuation of water level variation along the river was observed. In the near-dam reservoir tail reach, water level response was dominated by unsteady discharge from the studied hydropower station, whereas in the downstream reach, it was controlled by backwater effects from the downstream reservoir, showing a 6-19-fold spatial difference. [Conclusion] Water level variation can be limited to within 1 m by controlling discharge fluctuation to ≤800 m³/h, providing theoretical basis for coordinated cascade hydropower operation and downstream comprehensive risk assessment.