Ulan Suhai Lake is the largest lake wetland in the Yellow River Basin and an important natural hydroecology barrier. In order to find out the structure of phytoplankton community, the eutrophication status of water body and the influence of environmental factors in Ulan Suhai Lake, three sampling points were set up in July and September 2022 to investigate and monitor the phytoplankton and water environment indexes, and to analyze the lake eutrophication status and the environmental factors affecting phytoplankton. The results showed: (1) A total of 108 species (genera) of phytoplankton belonging to 8 phyla were identified through these two investigations. Chlorophyta and Bacillariophyta were the dominant species. A total of 7 dominant species were screened, and the advantages of Cyanobacteria were obvious, among which Pseudoanabaena and C.annulata were the absolute dominant species. (2) The average Shannon-Wiener diversity index was 2.15, and the water pollution degree in the center of the lake was higher than that in the south and north of the lake. (3) The lake eutrophication status was mild to moderate eutrophication. (4) The abundance of phytoplankton is affected by conductivity, total phosphorus and dissolved oxygen. It is recommended to strengthen the monitoring of total phosphorus and take targeted measures to further improve the hydroecological status of Ulan Suhai Lake. The study results can provide reference for the hydroecological protection and management of Ulan Suhai Lake.