Abstract: Representative effluent from sewage treatment plants in Dianchi Lake was selected to simulate and investigate the degradation process and spectral characteristics of COD under natural conditions, and to explore the influence of the effluent on the concentration of COD in Dianchi Lake. The results show that: 1) The COD in the effluent from sewage plants occurred mainly in dissolved states and flew directly into Dianchi Lake; the degradation rate of COD_Cr was 28.9% and was quite low under the experiment condition, and the degradation process accords with the equation C=-2.46ln t+29.018 (R ²=0.932 2). COD was easy to accumulate under such condition. The degradation rate of COD_Mn was 15.8%, and the degradation process accords with the equation C=-0.348ln t+7.473 (R ²=0.995 7). 2) UV Vis spectroscopy and three-dimensional fluorescence spectra characteristics show that the COD in the effluent from sewage plants was mainly in the category of fulvic acid. The structure of COD in the effluent from sewage plants was relatively stable in the process of degradation. The slightly increasing of aromatic part of COD further confirmed that most organic compounds in the effluent from sewage plants remained in the water and the COD would gradually accumulate if the water did not change timely. 3) With the increase of the sewage treatment capacity in the lake basin, the effluent from sewage plants had become the main source of the water quantity in Dianchi Lake. And as the COD in the effluent has the refractory characteristic, the increasing of the COD concentration in recent years was closely related to the discharge of effluent from the sewage plants.