Abstract: From December 2008 to October 2009, a seasonal investigation was conducted on the phytoplankton’s community structure and its relationships with environmental factors in Datong Lake. With the comparison of the historical data in 1960, the potential effects of intensive aquaculture on the aquatic environment were analyzed, aimed to provide theoretical support for the sustainable fishery of freshwater lakes. A total of 98 phytoplankton species belonging to 7 phyla and 54 genera were collected, among which, Peridinium bipes, Chroomonas acuta, Chlorella vulgaris, Crytomonas ovate, Cyclotella meneghiniana, Crytomonas erosa, Anabaena circinalis, Microcystis aeruginosa, and Anabaena azotica were the dominant species, and had obvious seasonal variations. The mean annual cell density of the phytoplankton was 1.84×10⁶ cells·L^-1, being the highest in summer (16.4×10⁶ cells·L^-1) and ranged from 1.71×10⁶ to 1.98×10⁶ cells·L^-1 in the other three seasons. The values of the abundance index, Shannon index, and Pielou index of the phytoplankton community were 2.01-4.55, 1.26-2.69, and 0.69-1.27, respectively. Canonical correlation analysis (CCA) showed that water depth, water temperature, transparency, and water total phosphorus content, oxidationreduction potential, and electrical conductivity were the main environmental factors affecting the phytoplankton community structure in the Lake.