In order to explore the structural characteristics of phytoplankton community in the main reservoirs in arid climate area, northwest China, and to further study the correlation between phytoplankton and environmental factors, 8 major reservoirs in different distribution areas were surveyed for 4 quarters in 2017. The results reveled that a total 294 species belonging to 8 phyla and 106 genera were identified to be mainly composed of the Bacillariophyta, Chlorophyta and Cyanophyta and in the proportions of 48.35%, 26.64% and 14.47%, respectively. During the survey period, the density of phytoplankton was between 0.3×10⁴-4.38×10⁷ cells/L, which was higher in Summer and Autumn than in Winter and Spring, with the existence of a significant increasing trend with decreasing altitude. The Shannon-Wiener diversity index, Simpson index, Pielou evenness index, and Margalef index were 2.26, 0.77, 1.20, and 0.86, respectively. The comprehensive evaluation combining the density of phytoplankton, indicator species and four diversity indices showed that K1, K2, K3 and K5 reservoirs were oligotrophic, while K4, K6, K7 and K8 reservoirs were meso-trophic to trophic. Spearman correlation analysis of phytoplankton density and environmental factors found that phytoplankton density was extremely significantly correlated with total nitrogen (TN), electrical conductivity (Spc), and altitude, and significantly correlated with temperature, pH, and Sal. Spearman correlation analysis and CCA analysis found that TN, Spc, temperature, and altitude were the main factors influencing phytoplankton density and dominant species distribution in reservoirs. Water-blooming cyanobacteria such as Merismopedia minima, Microcystis smithii and Microcystis wesenbergii had an absolute advantage in low-altitude reservoir as temperature rise. It was worth noting that the density of Microcystis smithii increased with temperature rise and exceeded the bloom threshold (10⁷ cells/L) in Summer.