Gehu Lake, which water system consists of Jintan and Wujin districts, Changzhou and Yixing cities, located at Jiangsu Province in East China. The dense population and well-developed industry and agriculture have led to heavy nitrogen pollution with complex nitrogen sources in Gehu Lake. Therefore, it is necessary to distinguish the contributions of each nitrogen source for nitrogen pollution control. In this study, domestic sewage, fertilizer and industrial waste water were employed as nitrogen end members. The end-member mixing model of nitrogen and oxygen isotopes showed that nitrogen contributions from sewage, fertilizer and industrial waste water were 22.79%, 33.25%, and 43.96%, respectively. Meanwhile, the budget analysis on government statistical data and quantified the absolute nitrogen load of different sources in 2013 disclosed that nitrogen contributions were 42.09%, 48.11% and 9.80%, respectively. These two methods characterize nitrogen contributions from sewage, fertilizer and industrial waste water in Gehu Lake at different time scales and provide scientific references for nitrogen pollution remediation.