Abstract: Nitrogen pollution is one of the most serious water environmental problems in watershed water pollution control. It is of great theoretical and practical significance to study the characteristics and sources of nitrogen pollution in typical agricultural watersheds in southeast China under the multiple impacts of human activities and climate change. To explore the nitrogen pollution pattern and pollution source of surface water in Jiulong River Watershed, this study carried out multi-point and multi-section in situ observation of surface water in the whole watershed during wet (July) and dry (January) periods of Jiulong River Watershed. Positive Matrix Factorization was used to analyze the sources and contribution rates of nitrogen pollution of surface water under different water seasons. Coupling correlation statistical analysis method was used to clear the key factors of nitrogen pollution of surface water under different water seasons. The results show that the characteristics of nitrogen pollution of surface water in Jiulong River Watershed had obvious water season differentiation. The concentration of TN ranged from 0.72-13.14 mg·L^-1, which was 1.39-10.95 mg·L^-1 in wet season and 0.72-13.14 mg·L^-1 in dry season. NO₃^--N, NH₄⁺-N, NO₂^--N and DON concentrations were higher in wet season than those in dry season, but PN concentration was higher in dry season than that in wet season. NO₃^--N, NH₄⁺-N and DON were the main forms of nitrogen pollution in wet season, and NO₃^--N, DON and PN were the main forms of nitrogen pollution in dry season. PMF model analysis of nitrogen pollution sources in surface water show that agricultural non-point source pollution, domestic sewage, biological residue, microorganisms and mineral particles in the river were the main sources of nitrogen pollution in the watershed during wet season, while agricultural fertilizer pollution, urban industrial sewage discharge and suspended micro-particle of biological residue in the river during the dry season were the main sources of nitrogen pollution in the watershed water. The results of the study can strengthen the understanding that nitrogen pollution of surface water in Jiulong River Watershed is affected by agricultural production activities and runoff changes. The study has also put forward the idea that the source control of nitrogen pollution from agricultural and industrial production and the water season differentiation of nitrogen pollution can be effectively brought into the comprehensive management of nitrogen pollution in Jiulong River Watershed.