Abstract: Guizhou Province experienced a severe provincial-wide drought from July 2022 to March 2023, which was the driest period in the past decade. In order to explore the phytoplankton community structure and its relationship with the habitat of an urban reservoir under extreme drought conditions, we selected Baihua Reservoir located in Guiyang City as the research site, and water samples were collected in three hydrological periods which were July 2022, November 2022 and March 2023. Niche breadth, niche overlap, the variance ratio method, the Chi-square test and association coefficients were used to analyze the niche and interspecific associations of dominant species. The results showed that a total of 7 phyla, 67 genera and 101 species of phytoplankton were identified in Baihua Reservoir, of which 5 phyla, 10 genera and 11 species were dominant species. Phytoplankton in all hydrological periods were dominated by Cyanophyta, and the proportion of Cyanophyta cell density exceeded 41.03%, Pseudanabaena limnetica was the dominant species throughout the year. Baihua Reservoir was an urban reservoir, the water quality indices and cell density of phytoplankton were obviously affected by different hydrological periods, the average values of electrical conductivity, pH, total dissolved solids, salinity, total nitrogen, total phosphorus, chlorophyll-a, and turbidity during the dry season were the highest among the three different hydrological periods. The niche breadths of dominant species varied greatly, ranging from 0.266 to 0.837, with the maximum and minimum values in P. limnetica in dry seasons and Microcystis flos-aquae in normal seasons, P. limnetica had a relatively large niche breadth (0.431-0.837) and strong ability to adapt to diverse habitats in different hydrological periods. The analysis of interspecific association showed the association of dominant species pairs were insignificant, indicating that the dominant species were independent in resource utilization and environmental adaptation. The cell density of phytoplankton in dry seasons was highest, while that in wet seasons was lowest. The dry season was characterized by low water temperature and slow growth of phytoplankton, weaker interspecific competition, and no niche overlap of dominant phytoplankton species. The suitable water environment at the end of the dry season, was conducive to the explosive growth of P. limnetica, the cell density of phytoplankton reached 3.04×10⁷ cells/L. Urban reservoir management should pay close attention to the abnormal proliferation of algae under extreme drought conditions. This study can provide a theoretical basis and data support for the early warning of algae blooms in Baihua Reservoir.