Taking Nanhudang Lake in Changshu City, Jiangsu Province as an example, the pollution discharge of non-point pollution sources, including agriculture, aquaculture, livestock, poultry breeding, and resident living sewage, was estimated. The allowable discharge of TN and TP in Nanhudang Lake was estimated and the eutrophication status in Nanhudang Lake was assessed. The results show that the current discharge of TN and TP are 113. 8 tons per year and 18. 3 tons per year, respectively, and the allowable discharge of TN and TP are 134. 4 tons per year and 6. 4 tons per year, respectively. The discharge of TN is close to the allowable discharge of TN and the discharge of TP goes far beyond the allowable discharge of TP. The water quality of Nanhudang Lake is in the transition phase from the water standard Ⅲ to water standard Ⅳ. According to the calculated eutrophication parameters, Nanhudang Lake is in the moderate eutrophication phase. The results can provide supports for the pollution control and management of Nanhudang Lake, and the research strategies and methods can provide reference for similar lakes.