Abstract: This study investigated the phytoplankton community dynamics in response to the catastrophic "23.7" flood in the Haihe River Basin in 2023. Sampling was collected at three stations along the Duliujian River (32 samples in total) to analyze phytoplankton species composition, abundance, biomass, and diversity indices, coupled with measurements of physicochemical water parameters. The mechanisms through which the flood event influenced the phytoplankton community were elucidated. Results indicate that the number of phytoplankton species was significantly higher during the flood (102 species) than after the flood (85 species). Cyanophyta and Chlorophyta were the dominant groups, their ecological advantage being closely associated with elevated nutrient levels and adaptability to hydraulic disturbance. After the flood, mean total suspended solids (TSS) increased, nitrate nitrogen (NO₃^--N) concentrations rose significantly (P < 0.05), and dissolved oxygen (DO) concentrations showed a significant negative correlation with TSS (P < 0.01). Redundancy analysis (RDA) revealed that during the flood, phytoplankton community structure was primarily driven by total phosphorus (TP), chlorophyll a (Chl-a), and ammonium nitrogen (NH₄⁺-N) concentrations, whereas after the flood, total nitrogen (TN) and Chl-a became the key drivers. This shift reflects a temporary change in nutrient limitation induced by the flood discharge. A comprehensive water quality assessment indicate that the Duliujian River was in a moderately polluted (β-mesosaprobic) state. This study provides a scientific basis for the management and protection of riverine aquatic ecosystems and offers theoretical support for addressing the impacts of extreme hydrological events.