Abstract: In order to understand the status of the ecosystem of the Heishui River Reservoir Bay after impoundment, a monthly survey of phytoplankton and water environment was carried out from June 2023 to May 2024. The results showed that the annual variation range of reservoir water level was from 773.51 to 824.16 m, with a total fluctuation of 43.64 m. The average water level during the impoundment period was 781.99 m, the high water level operation period was 807.02 m, the fade period was 800 m, and the low water level operation period was 773.54 m, which was significantly higher than that of the other three periods (P<0.001), and the fade period was significantly higher than that of the low water level operation period (P<0.05). During the survey period, the number of phytoplankton species showed a trend of S3>S2>S1. A total of 6 dominant species were found, including Pseudanabaena sp., Plagioselmis sp., Chrysochromulina parva Lackey, Cyclotella sp., Achnanthes sp., and Synedra acus var. Among them, Chrysochromulina parva Lackey became the dominant only during the impoundment period/fade period with significant water level fluctuations, and Pseudanabaena sp. became the dominant species during the high and low water level operation period with small water level fluctuations. The ecological type of phytoplankton community in the impoundment period was Chlorophyta-Bacillariophyta-Pyrrophyta type, the Chlorophyta-Bacillariophyta-Cyanophyta type in the high-water/low-water operation period, and the Chlorophyta-Bacillariophyta-Cryptophyta type in the fade period. The monthly mean values of cell density showed the high-water operation period> the impoundment period> the low-water operation period> the fade period, and spatially S3>S2>S1. The α diversity index showed that the temporal difference was larger than the spatial difference. There were significant differences in phytoplankton community structure at different water levels (R²=0.2059, P=0.003). Cell density and the α diversity index were negatively correlated with hydrodynamic parameters (water level, flow rate, and velocity). The phytoplankton community had obvious characteristics of water period. With the rise of water level and its fluctuation, the network complexity and dynamic stability of phytoplankton communities decreased. Water environment factors significantly responded to hydrodynamic changes, and there was a significant negative correlation among total nitrogen (P=0.0002), nitrate nitrogen (P=0.0002), ammonia nitrogen (P=0.011), turbidity (P=0.014), water temperature (P=9.50208E-05), dissolved oxygen (P=7.12609E-05) and redox potential (P=4.31393E-06), and conductivity (P=0.024) and transparency (P=2.46463E-07). The results confirm that hydrodynamic conditions have a regulatory effect on plankton communities and water environment. Managing hydrodynamic conditions can be used as an effective means to regulate algal communities and protect the water environment in reservoirs. This study provides a scientific basis for understanding the ecosystem status of the Heishui River Reservoir Bay and offers insights for ecological management of the Baihetan Reservoir.