Algae growth and nutrient concentration has a lower threshold for algae affected by nutrient concentration with geometric increase and an upper threshold for algae that is not affected by the increase of nitrogen and phosphorus concentration. However, the formation of cyanobacterial blooms is affected by many factors, the effects of nitrogen and phosphorus on cyanobacterial blooms vary greatly in different lakes, different areas, and different periods, making it difficult to determine the nitrogen and phosphorus control thresholds for cyanobacteria growth. Although studies on nitrogen and phosphorus thresholds for algal growth and control of cyanobacterial blooms have been carried out, they are mostly focused on laboratory studies or empirical judgments. Although there are also studies based on field-measured data, they are also limited to a specific area, while studies on nitrogen and phosphorus thresholds based on long series of field measurements and covering the entire lake are lacking. As a eutrophic shallow lake with a high nutrient background, the occurrence of cyanobacterial blooms in Lake Taihu is strongly influenced by nitrogen and phosphorus. Analyzing the temporal and spatial changes of total phosphorus (TP), total nitrogen (TN) and chlorophyll-a concentration (Chl.a) in Lake Taihu, it is found that TP, TN and Chl.a in the northwestern of the lake were significantly higher, and TP, TN and Chl.a are all positively correlated. To investigate the control thresholds of TP and TN for cyanobacterial bloom outbreaks in Lake Taihu, the Chl.a grading standard (10, 26] under the light eutrophication level was used as the condition to characterize the bloom outbreak, referring to the frequency distribution method put forward by Zheng Binghui, the TP and TN control thresholds were determined to be 0.05-0.06 mg/L and 1.71-1.72 mg/L, respectively. Through spatial verification, the concentration of TP and TN in the phytoplankton-dominated area of Lake Taihu is much higher than the control thresholds of TP and TN in the whole lake area under the same nutritional level. This indicated that the high nitrogen and phosphorus levels in the phytoplankton-dominated area provide sufficient nutrient basis for the occurrence of cyanobacterial blooms. Even if the average concentration of nitrogen and phosphorus in the whole lake is controlled below the threshold level, the risk of algal blooms in the phytoplankton-dominated area is still relatively high under suitable conditions of meteorology and hydrology. In addition, under the background of high nitrogen and phosphorus in the phytoplankton-dominated area, even in the low-risk seasons for cyanobacterial blooms that were once considered, the possibility of blooms is still high. In the past ten years, although Lake Taihu has undergone large-scale and high-intensity treatment, the nitrogen and phosphorus concentrations in the phytoplankton-dominated area of Lake Taihu are still at a high level because of the large proportion of the inflow pollution load from Huxi District around Lake Taihu Basin, which provides sufficient nutrients for the cyanobacterial blooms. In consequence, source control and emission reduction in Huxi District is still the key measurement for the prevention and control of eutrophication and cyanobacterial blooms in Lake Taihu.