Under the conditions of climate warming and water eutrophication, cyanobacteria blooms frequently broke out in Hulun Lake in 2022, covering almost the entire lake surface, destroying the water landscape and seriously threatening the health and safety of the lake ecosystem. In order to reveal the driving mechanism of large-scale outbreak of cyanobacteria in Hulun Lake, 13 sampling sites were selected to collect surface, middle and bottom water samples in spring, summer and autumn of 2022, and the species, cell density, biomass and water quality indexes of cyanobacteria were studied. The results showed that a total of 22 species of cyanobacteria were identified, and a total of 10 dominant species were identified, among which Microcystis sp. was the dominant species throughout the survey period. There were significant changes in the density and biomass of cyanobacteria in different periods and depths. The density (2.58×109 cells/L) and biomass (3.30×102 mg/L) of cyanobacteria in summer were 1~2 orders of magnitude higher than those in spring and autumn. In spring, the density and biomass of cyanobacteria were the highest in the bottom layer of the lake, and the highest in the surface layer of the lake in summer and autumn. The results of correlation analysis and redundancy analysis showed that there were seasonal differences in the influencing factors of cyanobacteria. Water temperature, nitrogen and phosphorus content, dissolved oxygen and pH were the key environmental factors affecting the occurrence of cyanobacterial blooms. In terms of control strategies, moderate control of nutrient content, implementation of nitrogen and phosphorus dual control, and improvement of cyanobacterial bloom prediction and early warning and emergency response capabilities are the fundamental ways to effectively reduce the risk of cyanobacterial blooms, which is of great significance for the prevention and control of cyanobacterial blooms in eutrophic lakes in the future.