To examine the relationship between phytoplankton and environmental factors during ice-sealed period in cold and arid regions, 15 sampling sites were set up in Lake Ulansuhai in January 2019．The structure characteristics of phytoplankton community and its related environmental factors were monitored. Pearson correlation analysis and redundancy analysis were carried out respectively to reveal the response of phytoplankton assemblages to environmental factors. The results showed that the winter season was characterized by the highest contribution of Pyrrophyta and Cryptophyta. The dominant phytoplankton groups in the different lake regions were varied. TheScontentSofStotalSphosphorus is the key environmental factor that affected the abundance and biomass of phytoplankton. The Lake Ulansuhai was at medium nutrient level in the period of the ice-sealed. This study enriched our understanding of the distribution characteristics of phytoplankton in eutrophic lakes, and provided an important guiding significance for evaluation and ecologicial remediation of water eutrophication.