Abstract: Nansi Lake is an important water transport channel and storage lake of the East Route Project of the South to North Water Transfer. Its water quality has been drawing wide public attention. This research investigates 14 monitoring sections in the main rivers flowing in and out the lake and 5 monitoring stations in the lake. The monitoring data is used to analyze the water quality variation trends of the rivers and the lake, including annual variation, intra-annual variation, different periods of time and different monitoring sites. The results show that the reduction ratio of COD_Cr and NH₃-N are 84.2% and 79.7% in 2007 compared with 2000 at the monitoring sections. The water quality in the Nansi Lake and its joined rivers has been greatly improved as a whole, but there are still large gaps compared with the target of consistently satisfying the water quality III standards. The water quality of the rivers and the lake has some fluctuation and is currently under eutrophication status, because of the effects of manifold factors, for example hydrology, sediment release, pollution source and the pollution control measures used in the basin. According to the current highlighted environmental problems, this paper proposes several appropriate countermeasures, such as calculating the distribution of the lake fishery capacity and the pollutant load capacity, paying attention to the control and management of non-point pollution sources, and promoting the strategy of restoring farmland to wetland and restoring fisheries to lakes. All of the above are urgent affairs.