The edge low-lying regions of are deep-water lakes when water levels rise and spread. We analyzed the changing characteristics of nitrogen stable isotope values of the particle organic matter and inorganic nitrogen during major flooding seasons in Bang Lake, one of the edge-lake of, and discriminated the origins and transformations of nitrogen pollution. The results showed that carbon and nitrogen stable isotopes value (δ¹³C, δ¹⁵N) of suspended particle organic matter in June were -26.7‰--23.7‰ and 2.6‰-6.2‰, respectively, which were in the range of those of soil organic matter (-25.21‰±0.52‰ and 3.79‰±0.37‰, respectively) and those of water plant (-28.8‰--24.9‰ and 5.3‰-8.2‰, respectively). The decreasing δ¹³C(-27.6‰--23.2‰) and the rising δ¹⁵N(4.3‰-7.7‰) in July relative to June showed more terrestrial detritus input besides soil organic matter and water plant when rainstorm washes. NH₄⁺-N was main form of inorganic nitrogen in June whereas NO₃^--N was main form of inorganic nitrogen in July. The more negative δ¹⁵NH₄⁺ (-18.6‰±5.2‰) showed ammonia-nitrogen mainly came from rain, and nitrate-nitrogen (δ¹⁵NO₃^-:1.4‰±3.0‰) mainly from agricultural fertilizer and rain. The concentrations and isotopic values of NH₄⁺-N and NO₃^--N had much more increasing (0.3 and 2 times, 6‰ and 3‰, respectively) in July than June, which were possible from livelihood wastewater and livestock and poultry breeding wastewater under rainstorm washing. δ¹⁵N of aquatic plant had more increasing in July (8.8‰±1.1‰) than June (6.6‰±1.1‰) due to aquatic plant absorbing high δ¹⁵N of wastewater. The mineralization and nitrification of nitrogen in the lake were strong by δ¹⁵N analysis, and alga made little contribution to internal nitrogen sources in flooding seasons. It was great threaten to watersheds pollution to livestock and poultry breeding along the bank of river and lake under rainstorm. This study can help to control nitrogen pollution of flooding lakes.