Although sporadic cyanobacterial blooms have been reported in many oligo-meso trophic waters, it is still unclear about the mechanism of cyanobacterial blooms forming in these waters. The study took Fangbian Reservoir in Nanjing as a case to identify the dominant bloom-forming cyanobacteria and analyze the primary driving forces affecting their dominance. The results showed that Planktothrix and Dolichospermum were the dominant cyanobaceteria which contributed to the occasionally occurring cyanobacterial blooms in the reservoir. Planktothrix was the species with greatest potential risks for blooming, which might occurred in July to September. Nutrient concentrations decreased significantly, in which total nitrogen decreased fast than total phosphorus. This situation was favor for cyanobacterial blooms control. However, fluctuation of nutrients, particularly the reduced C/P ratio, induced by the pulse compensation for phosphorus from surface runoffs in south catchment, might be the primary reason for those occasionally occurring cyanobacterial blooms in the reservoir. When nutrient demands for the growth of the two cyanobacteria was met, the dominance of Planktothrix were driven mainly by water temperature, COD_Mn and oxidation-reduction potential (ORP), and the dominance of Dolichospermum were driven by ORP, water temperature and transparency. These results were helpful to understand the forming mechanism of cyanobacterial blooms in oligo-meso trophic waters.