Phytoplankton, as the primary producers in lake ecosystems, influence material cycling and energy flow within water bodies. Their community structure and dynamic changes directly reflect the nutrient status and ecological health of the water body. To comprehensively understand phytoplankton variations and influencing factors in tailwater lakes of northern arid-cold regions, the representative Daihai Lake was selected as the study site. Water and sediment samples were collected across all four seasons in 2024 to systematically analyse phytoplankton community structure and its response to environmental factors. Results indicate that during the study period, water body total nitrogen (TN) and total phosphorus (TP) concentrations exceeded Class V and Class IV water standards, respectively, while sediment TN and TP contents surpassed the national sediment average. A total of 95 phytoplankton species belonging to 8 phyla were identified, dominated by Chlorophyta, Cyanobacteria, and Diatomeae. Phytoplankton cell density, biomass, and Chl.a concentration peaked in winter. Ten dominant species across 5 phyla were identified, primarily from Chlorophyta and Diatomeae, with Microcystis and Chromomonas serving as key dominant species throughout the year. Results from Shannon diversity index, Pielou evenness index, Margalef richness index, and Simpson dominance index indicated higher phytoplankton community diversity and greater ecosystem stability in spring and summer compared to autumn and winter. Phytoplankton communities in Daihai Lake exhibited low spatial heterogeneity, suggesting that diversity and richness are primarily influenced by seasonal variations. Statistical analysis indicates that water temperature (WT), pH, dissolved oxygen (DO), total nitrogen (TN), and dissolved inorganic phosphorus (DIP) are key factors influencing phytoplankton community structure. Multi-year surveys reveal that the dominant phytoplankton species in Daihai Lake have shifted from diatoms and green algae to cyanobacteria and cryptophytes. This study uncovers the key mechanisms driving seasonal succession in phytoplankton communities of northern cold-arid lakes, providing a scientific basis for ecological management and conservation of Daihai Lake.