Abstract: Polder is an important part of the landscape pattern in low-lying areas along lakes and rivers. Inorganic nitrogen pollution in surface water has become a serious environmental issue in the polder area. To identify the sources of nitrate pollution in the polder area, the Pingjiang River section of the Quyuan Polder of Dongting Lake was selected as the study area. Stable isotope and water chemistry analysis methods were used to qualitatively identify the pollution sources, and the MixSIAR model was used to quantify the contributions of different pollution sources. The results showed that: (1) Nitrate nitrogen and ammonia nitrogen were the main forms of inorganic nitrogen in surface water of the Pingjiang River section of Quyuan Polder. Temporally, there was no significant difference in nitrate nitrogen concentration between wet and dry seasons (p>0.05), while the average concentration of ammonia nitrogen value was higher in wet season than in dry season. Spatially, nitrate nitrogen concentration inside the polder was significantly lower than outside (p<0.01), while ammonia nitrogen concentration was significantly higher than outside (p<0.01). (2) Nitrogen and oxygen isotopes and the analysis results of the MixSIAR model confirmed that chemical fertilizers, soil organic nitrogen, aquaculture wastewater, manure and sewage were the main sources of nitrate in surface water of the study area. The contributions of these nitrogen sources to nitrate were 33.0%, 32.6%, 19.4% and 11.7%, respectively in the wet season, and 26.7%, 31.2%, 21.5% and 16.9% respectively in the dry season. Atmospheric deposition contributed only 3.5% of nitrate sources in surface water. (3) The nitrate transformation process in the study area was mainly nitrification, and no significant denitrification process was observed. The study shows that nitrate pollution in surface water of the study area is mainly affected by agricultural non-point source pollution, and the pollutants primarily come from soil organic nitrogen, chemical fertilizers, and aquaculture wastewater. Therefore, to prevent and control nitrate pollution, we should consider improving fertilizer efficiency and treating aquaculture wastewater.