Based on the previons study of the hydrodynamics of Dianchi Lake,a 2D COD_Mn water quality model was built up in this paper.By the model,the transport and the dispersion of effluents in Dianchi Lake were simulated and-analyzed.and the environmentsl effects of the Xiyuan Tunnel Project etc.on the C0D_Mn concentrations in Dianchi Lake were evaluated. The results of research are conductive to the water pollution controI of Dianchi Lake.