Analysis was carried out on the monitoring data of phytoplankton in the top layer of Lake Fuxian since 1980. It revealed that the phytoplankton community has been developing on both species diversity and total biomass in the past two decades. The species has increased from 45 to 78, and the biomass has increased 10. 5 times. The dominant forcing factor for the development was the accumulation of nitrogen and phosphorous in the lake, the retention radio of nitrogen and phosphorous was as high as 88.5% and 93% in the lake. The source of increased species was Lake Xingyun in the drainage basin of Gehe River, the largest inflow river of Lake Fuxian.