Based on the data of total nitrogen (TN), nitrate (NO-3), nitrite (NO-2), and ammonium (NH+4) mass concentrations in the middle and lower reaches of the Puyang River in Shaoxing City from 2022 to 2023, this study analyzes the primary forms, concentration levels, and spatial distribution of nitrogen pollution. The sources of nitrogen pollution were investigated by using both forward tracing methods (field monitoring, emission coefficient approach, and SWAT model) and reverse tracing methods (nitrogen and oxygen stable isotope methods).The results indicate that nitrogen pollution in surface water is predominantly in the form of nitrate nitrogen, with TN mass concentrations in the dry season significantly higher than that in the rainy season (p＜0.05). Higher nitrogen levels were observed in urban and agricultural areas compared to other regions. In 2022, domestic sewage sources accounted for the largest share of TN load (58.7%), followed by agricultural sources (39.5%). Areas with high TN load were concentrated along the densely populated lower reaches of the Puyang River and in subwatersheds with extensive farmland. There was a strong positive correlation between TN load and precipitation (p＜0.01), reaching the highest valuein June during the rainy season. The degree of urbanization and residential clustering significantly influenced δ15NNO-3 values, with domestic sewage identified as the primary source of nitrate in the watershed (averaging 54.5%). The sources of nitrate in different tributaries also vary, driven largely by land use types and urbanization levels.