The water environmental capacity of phosphorus and nitrogen is an important factor to control water eutrophication in lakes/reservoirs. The quantitative connection between the growth of predominant species i. e. uniol-gat strain of Microcystis aeruginosa and the concentration of TP and TN in Lake Songhua was studied in the laboratory using simulation method. Results indicated that TP threshold is 0.065 mg/L, TN threshold is 0.843 mg/L in the Lake Songhua. Risk assessment theory and Monte Carlo method were employed to analyze the concentration distribution regulation of TP and TN in the lake based on the data from 2002 to 2004. The paper developed an uncertainty method to calculate water environmental capacity of TP and TN in lakes/reservoirs. Because the uncertainty method was based on water self-purification law, and it isn * t needed to calculate some parameters difficult to identify, such as the degradation coefficient of the pollutants. It is useful to calculate the water environmental capacity of TP and TN in the Lake Songhua with the uncertainty methods. The result indicated that the maximum permissible discharge amounts of TP and TN in the Lake Songhua watershed were 2123. 78 t/a and 7018. 82 t/a. In order to control that eutrophication probability in the Lake Songhua was below 0.001, amounts of TP and TN were need to decrease 3208.34 t/a and 18648.91 t/a, respectively.