Abstract: This study is to investigate and monitor the pollution situation of water quality of water substance in South Lake, the draining situation of drain outlets around South Lake, the flow and water quality of sewage The quantity and pollutant burden of sewage that drained into South Lake by drain outlets can be got. After the quantitative analysis on the point source pollution in South Lake, the Priod study can be provided for the comprehensive treatment to stop pollution or after pollution. The self-purification rules of pollutants in the drain outlets are analyzed and water quality model is built to simulate and calculate the distribution of concentration field of pollutants in water area round the drain outlets. Those provide a theoretical foundation to analyze the rules of concentration changes of pollutants in the South Lake. The Longwangzui sewage plant outlet is a bigger sewage outlet of the South Lake, the amount of sewage is more than 150000 tons everyday. The water quality model of water body near the Longwangzui sewage plant is based on the sewage characteristic analysis. The sewage pollutant is divided into two parts: settled matter and unsettled matter. The self-purification of settled matter in the water body is sedimentation and the unsettled’s is diffusion by laminar flow and dilution by turbulence flow. The two parts enter into lake and mix with the water in lake altogether. The comprehensive self-purification coefficient of TN and TP are deduced by the water quality monitor data near the outlet and the coefficient of TN and TP is 462.24, 172.8 day-1 each.