Planktonic algae were investigated and researched monthly from January 2014 to December 2015 in Qingcaosha Reservoir, an important water source in Shanghai. There were 25 dominant species in 2014, and only 12 dominant species in 2015. Microcystis incerta, Pseudoanabaena sp., Cyclotella sp., Ankistrodesmus falcatus var. mirabilis and Klebsormidium scopulinum were common dominant species arising in two years. Compared with the data of 2014, the dominant species were substantially reduced in 2015, and the seasonal variation of dominant species was less obvious than in 2014. As a whole, the dominant species were more in summer and autumn than in winter and spring. The majority of dominant species groups was Cyanophyta and Chlorophyta. Dominant species groups showed the characteristics of more Cyanophyta in summer, autumn and more Chlorophyta in spring, summer. Algae of Bacillariophyta and Xanthophyta became dominant species more likely in spring and summer. Redundancy Analysis showed that water temperature, dissolved oxygen, total nitrogen, total phosphorus were the main environmental variables affecting most dominant species density. As for Microcystis incerta, the important dominant species, its density was positively related with water temperature. As for Tribonema sp. and Cyclotella sp., their density had negative correlation with water temperature.