[Objective] The source characteristics of nitrate pollution in the high and low water period of Fenhe River were analyzed and studied in order to provide theoretical support for the treatment of nitrogen pollution in the lower reaches of the Fenhe River.[Methods] The changes of nitrate and its nitrogen and oxygen isotopes in different periods in the lower reaches of Fenhe River in 2018 was analyzed to investigate the characteristics of the change of nitrate sources discharged into the river in rainy periods. And the Isosoure model was introduced to calculate the contribution of each pollution source to the nitrate in the lower reaches of Fenhe River.[Results] In the high and low water period, the variation range of δ¹⁵N was 3.45‰~11.19‰, δ¹⁸O value was -0.72‰~3.17‰. The nitrate pollution sources were mainly agricultural fertilizer, soil organic nitrogen, domestic sewage and feces. The nitrate pollution sources were mainly related to the land use types around the lower reaches of Fenhe River. In the high water period, the nitrate pollution sources at Linfen section and M₅ sampling point were mainly feces and sewage, accounting for 58%, 40%, 72% and 58%, respectively. The contribution rate of agricultural fertilizer in M₆-M₉ section was the highest, accounting for 45.4%, 62%, 56.6% and 56.5%, respectively. The highest contribution rate of nitrate pollution sources in all sampling points in low water period was feces and sewage, accounting for 40%~73%.[Conclusion] The main sources of nitrate pollution in the runoff of Fenhe River are agricultural fertilizer, soil organic nitrogen, domestic sewage and feces, and the pollution sources are closely related to land use types. Most of the sources of nitrate pollution in different water periods are feces and domestic sewage, and the difference is mainly reflected in the M₆-M₉ sampling point. The main pollutants in the wet period are agricultural fertilizer, while in the dry period are feces and domestic sewage.