Algal bloom, caused by abnormal proliferation of algae, is one of the major ecological disasters threatening the safety of many drinking water resource reservoirs in China. Investigating the driving factors of algal proliferation in drinking water resource reservoirs is a prerequisite for preventing and controlling algal blooms and water quality problems. To investigate the driving factors of algal blooms in reservoirs, the cell density and biomass of phytoplankton and related environmental factors were analysed during 12 months from Fushi Reservoir and Laoshikan Reservoir in the Taihu Basin. The results showed that reservoirs in the subtropical monsoon region were at risk of proliferation of cyanobacteria and diatoms. A severe diatom bloom dominated by Achnanthes occurred in Laoshikan Reservoir in May, and the corresponding cell density of diatom reached 6232×10⁴ cells/L. Cyanobacterial blooms occurred in June and September in Fushi Reservoir. The result of redundancy analysis indicated that total phosphorus, water temperature and water level were key factors influencing the variation of phytoplankton community structure in reservoirs. A total phosphorus concentration higher than 0.024 mg/L could promote algal proliferation. The risk of cyanobacteria and diatom blooms increased significantly when the water temperature was higher than 20℃ and 16℃ respectively, while the best water temperature for cyanobacteria and diatom blooms was 30℃ and 22℃ respectively. Low or drastic fluctuations in water level could be beneficial for algal blooms. Our results indicate that the abnormal proliferation of phytoplankton in mesotrophic drinking water reservoirs is influenced by nutrient, hydrological and meteorological factors. External nutrient loading should be reduced to prevent and control algal blooms in such mesotrophic reservoirs, and extreme weather events, hydrological processes and food chain regulation should also be considered.