Abstract: The lake water quality model based on the mass balance with an assumption of well-mixed condition was introduced.The parameters were inferred and the model verified by the monitoring data on Dianchi Lake,and the application of the model in the projection of the water quality in Dianchi Lake was given.The adaptability of the assumptions for the model to Dianchi Lake was finally discussed.It was shown that the model could be applied in the projection of the long-term concentration of the organic pollution in Dianchi Lake.