Abstract: Liujiaxia Reservoir is an important conservancy and hydroelectric project in the Yellow River which is the second largest river in China. With Liujiaxia Reservoir as the second water source of Lanzhou, water qualityhave drawn more and more attention from the government and researchers. Phytoplankton as the main primary producer, plays an important role in aquatic food, web ecosystem, and is a sensitive environmental indicator of water quality in lakes and reservoir. In this study, to understand the phytoplankton status and the nutritional conditions of Liujiaxia Reservoir, eleven sampling sites were established across the reservoir. We conducted 8 times qualitative and quantitative phytoplankton acquisitions from 11 sampling sites in Liujiaxia Reservoir. To analyze the phytoplankton community structure during March to October in 2020 and discussed the impact of water level and other possible influencing factors on the phytoplankton community structure, a total of 63 species of phytoplankton were identified, which belonged to 35 genera and 7 phyla. Among which, the most abundant group was Bacillariophyta, with 40 species in 17 genera, followed by Chlorophyta (12 species in 8 genera), Cyanophyta (4 species in 4 genera), Pyrrophyta (2 species in 2 genera), Xanthophyta (2 species in 2 genera), Euglenophyta (2 species in 1 genera), Chrysophyta (1 species in 1 genera). The dominant species were Asterionella formosa, Fragilaria crotonensis and Synedra acus, they all belong to Bacillariophyta. The density of phytoplankton varied from 3.17×10⁴ ind./L to 59.09×10⁴ ind./L, with an average density of 13.54×10⁴ ind./L. The biomass of phytoplankton varied from 0.0370 mg/L to 0.5004 mg/L, and the average biomass was 0.2137 mg/L. The indices of Shannon-Wiener H′, Margalefrichness index D and Pielouuniformity index J were 1.16—2.43 (averaging 1.84), 1.22—1.57 (averaging 1.42), and 0.23—0.51 (averaging 0.38), respectively. Due to release flood waters during flood season, phytoplankton community structure of Liujiaxia Reservoir did not change regularly, the results indicated that artificial regulation and storage had obvious effects on phytoplankton community structure. The phytoplankton in Liujiaxia Reservoir are mainly diatoms and Chrysophyta, indicating that it is a poor nutritive water body. RDA indicated that the most influential environmental factors on phytoplankton were total nitrogen and transparency, followed by dissolved oxygen and water temperature. Water quality results show except for the total nitrogen, the other water quality indexes of Liujiaxia Reservoir meet the water quality standards of class Ⅰ or class Ⅱ of the National Surface Water Quality Standards (GB3838-2002). This study provides background data for water environment protection, rational utilization of fishery resources and sustainable development of Liujiaxia Reservoir.