[Objective] In response to the significant disturbance of unsteady flow processes in downstream rivers caused by the operation of super-large hydropower stations, this study aims to reveal the propagation mechanism and energy dissipation law of hydraulic fluctuations in the fluctuating backwater area immediately downstream of the dam under the context of cascade reservoir development. By analyzing the key driving factors and spatiotemporal evolution characteristics of hydraulic fluctuations under complex boundary conditions, it provides a theoretical basis for optimizing cascade reservoir operation, ensuring project safety, and integrated river management downstream. [Methods] The study focuses on an approximately 38 km long reach within the fluctuating backwater area downstream of a super-large hydropower station. Thirteen monitoring sections (SD1-SD13) were established along the main channel. Multi-parameter synchronous prototype observations were conducted during five typical periods covering storage, drawdown, and flood seasons under various operational scenarios (different discharge rates: 2 000-8 000 m³/s; different downstream pool levels: 562-596 m). High-precision water level gauges and Acoustic Doppler Current Profilers (ADCP) were used to collect continuous water level and cross-sectional velocity data. Through spatiotemporal comparative analysis and a multiple linear regression model, the propagation characteristics of hydraulic fluctuations (peak attenuation, velocity distribution) and the weighting of main influencing factors (upstream discharge, downstream backwater effect, channel topography) were revealed. [Results] (1) The propagation of hydraulic fluctuations is jointly regulated by upstream discharge, downstream backwater effect, and channel topography. The peaks of water level and discharge downstream of the dam are significantly synchronized. Peak attenuation shows spatial differentiation: the attenuation rate is higher in the near-dam reach (0-16.5 km) due to strong turbulence, reaching up to 0.170 m/km under high discharge; it slows down in the far-dam reach (16.5-37.7 km) under the combined effects of downstream backwater and channel widening. (2) Velocity distribution exhibits both longitudinal differentiation and local abrupt changes. Under extreme flood discharge conditions (>6 000 m³/s), a super-strong velocity zone (>3.0 m/s) forms within approximately 6 km downstream of the dam. Near the SD7-SD8 sections (about 24 km from the dam), the coupling effect of secondary flow and a deep thalweg (maximum drop of 20.3 m) causes a leftward shift of the main flow line and a local velocity surge. (3) Weight analysis of influencing factors indicates that water level fluctuations in the near-dam reach (e.g., SD1) are primarily controlled by upstream discharge (weight >0.94). The influence weight of the downstream backwater effect gradually increases with propagation distance, becoming dominant in the far-dam reach (e.g., SD9, weight>0.62). [Conclusions] The propagation and energy dissipation of hydraulic fluctuations in the fluctuating backwater area downstream of a super-large hydropower station are complex processes resulting from the dynamic coupling of discharge magnitude, channel topography, and downstream pool level. Fluctuation energy is mainly dissipated through strong turbulence in the near-dam reach, while its propagation in the far-dam reach is significantly constrained by downstream backwater and channel storage effects. The propagation of hydraulic fluctuations on the surface is the result of multi-factor coupling of flow energy level, channel topography and downstream water level.