A new cyanobacteria bloom forewarning method was proposed in this paper based on the qualitative and quantitative study of phytoplankton in 5 sites of Meiliang lake in summer. The advantage of semi-quantitative vivo monitoring method was able to identify the species of cyanobacteria blooms promptly and accurately. By comparing with the Lugol's solution fixed method in qualitative and quantitative analysis of phytoplankton, the vivo monitoring method exhibited excellent reliability and stability in identifying the dominant species and obtaining dominance.