Accurate monitoring of cyanobacterial blooms is the basis of lake eutrophication research, which can provide a basis for the management and decision-making of water environment departments. In this paper, Dianshan Lake is chosen as the research object, Sentinel-2/MSI and Landsat-8/OLI image data are used to monitor a total of 21 algal bloom areas in Dianshan Lake from 2019 to 2021 through the improved floating algae index (FAI). The distribution of eutrophication in Dianshan Lake is also analyzed. The results show that cyanobacteria blooms and eutrophication have appeared in Dianshan Lake for three consecutive years. The high incidence period of cyanobacteria blooms in this lake is from June to September each year. The area of eutrophication is large and the frequency is high. In September 2020 and August 2021, the bloom area exceeds 20km2.The blooms burst quickly and last for a short time.