Abstract: To deeply investigate the influence mechanism and regulatory patterns of hydrodynamic conditions on the occurrence and recession process of abnormal algal proliferation in large reservoirs, this study takes the abnormal algal proliferation event in the Danjiangkou Reservoir during the summer of 2025 as a typical case. A two-dimensional planar hydrodynamic-water age coupled model was constructed based on the EFDC model. The model simulated and analyzed the characteristics of the flow field structure, the intensity of water exchange, and the dynamic spatiotemporal distribution of water age in the reservoir area during the algal proliferation event. The research results indicated that during the low-water-level operation period, the Danjiang sub-reservoir exhibited overall characteristics typical of a static lake with an average flow velocity in the Danjiang sub-reservoir below 0.01 m/s and strong water retention, and the water age continued to accumulate and increase over time, creating a stable environment conducive to the rapid proliferation of algae. Upon the arrival of the autumn flood season, the substantial inflow of floodwater significantly altered the original hydrodynamic pattern. Through intense water mixing, it promoted water exchange between the Hanjiang sub-reservoir and the Danjiang sub-reservoir, with the maximum instantaneous exchange discharge reaching 6 015 m³/s. This hydrodynamic process effectively reduced the water age in the Danjiang sub-reservoir, disrupted the original hydrodynamic conditions necessary for algal aggregation, and thereby significantly accelerated the recession of the abnormal algal proliferation event. The research findings reveal the reshaping effect of autumn flood events on the hydrodynamic conditions of reservoirs and analyze the impact of strong hydrological disturbance events on the occurrence and recession process of abnormal algal proliferation in the reservoir. The results can provide a reference basis for early warning of abnormal algal proliferation and water ecological environment management in large reservoirs.