To investigate the water characteristics and phytoplankton status of Yunmeng Lake, two sampling surveys were conducted in August and October 2014. The results showed that the total nitrogen （TN） ranged from 0.494 mg·L^-1 to 8.215 mg·L^-1, the total phosphorus （TP） ranged from 0.029 mg·L^-1 to 0.053 mg·L^-1, the chlorophyll a （Chla） concentration ranged from 13.65 mg·L^-1 to 61.05 mg·L^-1 and the permanganate index（COD_Mn） ranged from 6.51 mg·L^-1 to 11.08 mg·L^-1 in Yunmeng Lake. The comprehensive trophic level index（TLI）of the entire reservoir area was relatively uniform, ranging from 57.85 to 69.69, with little variation. A total of 60 species of phytoplankton belonging to 8 phyla were identified in Yunmeng Lake, with higher species richness in October （46 species） than in August （39 species）. Chlorophyta was the most diverse phylum, and the species composition was relatively richer at the inlet and outlet of the reservoir. The dominant species included Phormidium sp., Oscilatoria lemmermanni, Anabaena constincta, Synedra sp. and Cryptomonadaceae erosa. Correlation analysis showed that phytoplankton density （D）, Shannon-Wiener diversity index （H），species number （S） and evenness index （J） were significantly correlated with dissolved oxygen （DO） （P<0.05）. Redundancy analysis（RDA） indicated that NH^＋₄-N，Chla，and COD_Mn had significant influences on the distribution of dominant phytoplankton species and changes in community structure. DO，NH^＋₄-N，and Chla were identified as important limiting factors for the phytoplankton species composition in Yunmeng Lake （P<0.05）.