The investigation in 2007 revealed that Guanting Reservoir had been a eutrophic lake. The concentrations of total phosphorus (TP), total nitrogen (TN) and salt permanganate index (CODMn) were 0.12mg?L-1, 1.63mg?L-1 and 601mg?L-1 respectively. The density of phytoplangkton was 1794.08×104cells L-1 including 51.57% of Cyanophyta, 2512% of Chlorophyta and 12.56% of Bacillariophyta. The water bloom of Microcystis happened in summer-autumnal seasons. The microcystin (MC-LR) was 0.794μg?L-1 in average concentration, but it was increased to 1.231μg?L-1 in August and September in 2007, which was gone beyond 23.1％ in safety limitation value of drinking water on the earth′s surface. The MC-LR content was positive correlation with Cyanobacteria density in Guanting Reservoir. The statistical test results showed that TP and NO3-N were main nutrient factors influencing the biosynthesis MC-LR. However, solubility iron was main microeiement influencing the biosynthesis MC-LR. Therefore, managing eutrophic lake had been a key for reducing the microcystin content and recovering the function of drinking water source in Guanting Reservoir