Data of 20 year monitoring of water quality, algal bioassay, and remote sensing information were analyzed to determine the long-term effect of nutrients (N and P) on algal growth. Since the first algal bloom in 1985 and the accumulation of nutrient, Lake Dianshan has become hyper-euthophicated. The concentrations of TN, TP and Chlorophyll have rapidly increased since 1999, and their annual rates were 2.25, 6.67 and 3.40 times respectively higher than that of the past 15 years, among which phosphorus was the highest. The transparency decreased 5cm annually. Algal community has been transited to green-bluegreen algae dominated since 1999, the frequency of algal bloom occurrence were 2-3 times of before. When concentration of TN in water column was higher than 3.5mg/L, AGP test did not have any positive response at any significant levels. In situ experiment in 2002 and regression analysis of frequencies between algal bloom and higher concentration of TN found that during the period of summer-autumn, algal growth could be increased when TN was around 3.5mg/L: the higher TN concentration, the more frequency algal bloom. The evidence showed that Lake Dianshan was of high risk of harmful bluegreen algal bloom.