Lake ecosystems in cold and arid regions are highly sensitive to climate change and anthropogenic disturbances, with phytoplankton community dynamics serving as a key indicator of aquatic ecological changes. Hulun Lake, a typical shallow lake in the arid and semi-arid region of northern China, has experienced significant alterations in its hydrological cycle and pollution patterns following the implementation of the "River Water Diversion to Lake" project, leading to eutrophication. Against this backdrop, investigating the assembly mechanisms of its phytoplankton community and identifying the key environmental drivers are crucial for lake ecological restoration and algal bloom prevention. This study conducted systematic ecological surveys at 26 sampling sites in the main body of Hulun Lake and its major inflow rivers during spring and summer of 2024 and 2025. The aim was to analyze the seasonal succession patterns of the phytoplankton community, its taxonomic beta diversity characteristics, and its coupling relationship with environmental factors. The results showed that: The phytoplankton community structure exhibited significant seasonal succession. Bacillariophyta dominated in spring, with Cyclotella menaquinone as the key dominant species, while the community shifted to absolute dominance by Cyanobacteria in summer, led by Microcystis aeruginosa and Anabaenopsis oscillarioides. Although species richness was higher in summer, the Shannon-Wiener diversity index was significantly lower than in spring. Canonical Correspondence Analysis (CCA) indicated that the key environmental drivers of community succession differed between seasons. The spring community was primarily influenced by Total Phosphorus (TP), Turbidity (Turb), and Chemical Oxygen Demand (BOD₅), whereas the summer community was mainly driven by Water Temperature (WT), Total Nitrogen (TN), Five-day Biochemical Oxygen Demand (BOD₅), and Chlorophyll-a (Chl-a). Decomposition of taxonomic beta diversity revealed that the total β-diversity between seasons and across years was primarily driven by the turnover component, with a limited contribution from the nestedness component. This indicates that species replacement is the core process structuring the phytoplankton community in Hulun Lake, reflecting strong environmental filtering and habitat heterogeneity. This study, from the dual perspectives of species composition and community assembly mechanisms, reveals that environmental filtering is the core driving force shaping the seasonal dynamics of the phytoplankton community in Hulun Lake. The research findings not only identify key regulatory factors in the eutrophication process of Hulun Lake but also provide important scientific evidence and a case study for the aquatic ecological health assessment, cyanobacterial bloom early warning, and ecological restoration practices for Hulun Lake and similar lakes in cold and arid regions of northern China.