The security of water resources and water environment in the Yellow River Basin of Qinghai Province is of great strategic significance to the whole Yellow River Basin and the northern region of China. In order to understand the characteristics of nitrogen pollution in this region, water quality was sampled and monitored for 30 sections of the Yellow River, Huangshui River and Datong River in different water periods. The temporal and spatial distribution characteristics of water nitrogen forms were systematically analyzed. Pollution contribution ratio of main pollution sources was quantitatively calculated by using SIAR isotope model. The results show that: ① the total nitrogen concentration in water of this area ranges from 0.33 to 13.50 mg/L, with an average concentration of 3.40 mg/L, which is manifested as Huangshui River > Datong River > Yellow River, NO3-
-N. ② The concentration of total nitrogen shows wet period ≈ dry period > normal period in the Yellow River, wet period > normal period > dry period in Huangshui River, and wet period > normal period ≈ dry period in Datong River. The distribution of nitrogen forms in the three rivers were also different in different water periods, indicating that there might be different pollution sources for different water periods. ③ The analysis results of SIAR isotope model show that the contribution ratios of NO3-
-N of six sources in the three rivers were: soil source > litter source > industrial waste water source > rainwater sewage source > domestic sewage source and manure sewage source. And the average contribution ratios of soil source and litter source of Huangshui River decreased in wet season, while the contribution of industrial waste water source increased. Thus it is necessary to focus on the control of non-point source pollution input such as litter source and soil source, and point source input of industrial wastewater.