To reveal the phytoplankton community structure in the eutrophic lake water during the icesealed period in cold and arid regions and its response relationship with water quality indicators, this study took Ulansuhai as the object, and set up 12 sampling points in the lake area in January 2019 to collect water samples and phytoplankton. Through the qualitative and quantitative detection of phytoplankton and the determination and analysis of physical and chemical properties of water bodies, we clarified the spatial variation characteristics of phytoplankton community structure and the distribution law of main water quality indicators in Ulansuhai during the icesealed period; combined with RDA analysis and Pearson correlation analysis, we revealed the response relationship between phytoplankton and water quality indicators, which provided a theoretical basis for the assessment and prevention of water eutrophication. The results show that: (1) the water quality indicators of 12 sampling sites in Ulansuhai during the icesealed period were significantly different, and the water quality indicators had different trends from north to south. (2) A total of 61 species of phytoplankton were detected in Ulansuhai during the icesealed period, among which Cryptophyta had the highest abundance (4.76×10⁶ cells·L^-1), and Pyrrophyta had the highest biomass (18.09 mg·L^-1), but the dominant phytoplankton groups at different locations in the lake area were different, and the abundance of Chroomonas sp. and Pseudoanabaena sp. in the North Lake area was significantly higher than that in the South Lake area. (3) There are obvious differences in the spatial distribution of different species of phytoplankton, and the diversity of the P3 sample site is the lowest, and the diversity of the P8 sample site is the highest. (4) The abundance of phytoplankton was significantly positively correlated with TP, the biomass was extremely significantly positively correlated with TP, and the diversity index was positively correlated with TP. Studies have shown that the Ulansuhai water was at a moderate nutrient level during the icesealed period, and the total phosphorus (TP) content in the water body was the main factor affecting the species abundance and biomass of phytoplankton; The distribution characteristics of plants were that Cryptophyta, Cyanophyta and Pyrrophyta were dominant in the North Lake area; Chlorophyta was the most abundant in the South Lake area.