Abstract: Based on the water quality investigation in May(dry period) and October(wet period) 2009,the spatio-temporal variations of water quality and nutritional status in the large,shallow and subtropical Yangcheng Lake were analyzed,and the reasons for the differences in water quality and the emergency of eutrophication were discussed.The results are as following: the concentrations of main water quality parameters including TN,chlorophyll,TP and CODMn decrease gradually from the western part to the eastern part of the lake consistently,and TP in wet period is higher than in dry period while the others are opposite.The water quality are all classified as inferior Class Ⅴexcept of the central area of eastern part which is in Class Ⅳ with TN and TP acting as the primary pollutants though the whole lake in both periods.Although the trend of eutrophication increase has been eased,however,the water body is still in eutrophic state.The major source of pollution nitrogen and phosphorus analysis suggests that the concentrations of NO₃^--N and NO₂^--N are significantly positive correlated with TN in both periods,and PO₄^3--P and TP concentrations also shows significantly positive correlation in wet period.Additionally,there are significantly positive linear relationships between the content of Chl-a and TN only in dry period,while the correlation between Chl-a and TN and TP concentrations is not significant in both periods.Farming activities and sediments releasing may contribute to the above characteristics in Yangcheng Lake.