Ecological investigations on the community structure of phytoplankton in 10 large reservoirs of Guangdong Province, which played a great role in water supply, were conducted in January and July, 2012. A total of 207 species of phytoplankton were identified, belonging to 104 genera of 8 phyla, which were classified into 30 phytoplankton functional groups. According to the cluster analysis, the ten reservoirs were separated into four ecological regions. Ecological Region One included Nanshui and Xinfengjiang reservoirs, where dominant groups were Oligotropher-adaptive groups. Ecological Region Two included Feilaixia, Baipenzhu and Fengshuba reservoirs, where dominant groups were Mesotropher-adaptive groups and broad-spectrum groups. Additionally, Ecological Region Three and Four contained Gaozhou, Gongping, Dashuiqiao, Hedi and Tangxi reservoirs, where Gaozhou and Gongping reservoirs were characterized by Eutropher-adaptive groups and broad-spectrum groups, while other reservoirs were dominated by Eutropher-adaptive groups. The representative phytoplankton functional groups were significantly different among four regions, and corresponded well with water nutrient conditions.