Phytoplankton in cold-region lakes are key carriers of material cycling and energy flow in lake ecosystems. Compared with phytoplankton in lakes at mid-to-low latitudes, their community structure exhibits higher sensitivity to climate change and anthropogenic stress. To elucidate the spatio-temporal characteristics of phytoplankton community structure and its key driving forces in cold-region lakes, this study used samples from January, May, July and September to represent winter, spring, summer and autumn, respectively. Plankton community and water quality samples were collected from Lake Chagan in 2024 and subjected to laboratory analysis.The results showed that the water trophic state of Chagan Lake was mesotrophic in spring, summer, and winter (34.20≤TLI≤47.16), while it was eutrophic in autumn (48.85≤TLI≤59.56). A total of 175 phytoplankton species belonging to 8 phyla were identified, with Bacillariophyta having the highest biomass and Cyanophyta the highest cell density. There were 12 dominant phytoplankton species from 5 phyla; the absolute dominant species (with the highest dominance) in spring, summer, autumn, and winter were Synedra acus (Bacillariophyta), Tribonema ulothrichoides (Xanthophyta), Dolichospermum spiroides (Cyanophyta), and Synedra acus var. angustissima (Bacillariophyta), respectively. The Shannon-Wiener diversity index, Simpson diversity index, Pielou evenness index, and Margalef richness index of the phytoplankton community were generally low, indicating weak resistance to external and internal environmental changes. Based on Pearson correlation analysis, redundancy analysis (RDA), and Mantel tests, it was clarified that WT, TP, TN, NH??-N, NO??-N, and DO were the main controlling factors affecting phytoplankton cell density in Chagan Lake. Among these, WT, TN, TP, and NO??-N were the main factors influencing phytoplankton dominance, while TN, NH??-N, and DO dominated changes in phytoplankton community structure. Notably, low temperature, low light (ice cover) and weakened hydrodynamic mixing in winter and spring promoted Bacillariophyta to become the absolute dominant species, whereas high TN and NO??-N in summer and autumn facilitated Xanthophyta and Cyanophyta to become the absolute dominant species. To effectively prevent Cyanophyta from becoming the absolute dominant species across all seasons in Chagan Lake, it is urgent to reduce the concentration of nutrient salts (TN and NO??-N) input in summer and autumn to address the threats posed by climate warming. This study reveals the driving mechanism of phytoplankton community structure in northern cold-region lakes, providing a scientific basis for the aquatic ecological protection of Chagan Lake and similar cold-region lakes.