Abstract: Abstract: As one of the rivers with the highest elevation in the world, the Yarlung Zangbo River concentrates more than half of the water resources in Tibet. With the development of cascade hydropower stations in the Yarlung Zangbo River, human activities have changed the ecological environment and affected the nitrogen cycle in the river. In order to clarify the distribution characteristics of nitrogenous compounds and quantitatively analyze their sources, eleven sampling points were set up in the middle and lower reaches of the Yarlung Zangbo River and its main tributaries (Nianchu River, Lhasa River and Niyang River), and water samples were collected in March (the dry season) and August (the wet season) of 2018. The concentrations of dissolved oxygen (DO), total dissolved nitrogen (TDN), NO3-, NO2- and NH4+ were measured. Dissolved organic nitrogen (DON) was calculated as the difference between TDN and inorganic N. The occurrences and spatial-temporal distribution of nitrogenous compounds in the water samples of the rivers were analyzed, and the main sources of NO3- in the wet season were determined by the dual isotope analyses. The results showed that the DON was the main form of TDN. The concentrations of TDN ranged from 1.3 to 2.5 mg/L in the dry season and from 1.1 to 2.0 mg/L in the wet season, and showed a downward trend along the river. The NO3--N was the main form of inorganic nitrogen in the water, with an average proportion of 25.6% in TDN, and NH4+-N accounted for 17.5% in TDN, while NO2--N was not detected at the most sampling sites. The results of nitrogen and oxygen isotope analyses showed that the values of δ15N-NO3- ranged from -9.17‰ to 22.68‰, and δ18O-NO3- from -13.25‰ to 8.24‰ in the wet season. The NO3- mainly came from rainfall and fertilizer, organic nitrogen in soil, and sewage and feces. Based on the Stable Isotope Analysis in R (SIAR) model, the contribution rates of nitrate from different sources were quantitatively calculated. In the Shigatse and Lhasa sections, NO3- in the water body mainly came from the organic nitrogen in soil and the NH4+ in rainfall and fertilizer, with more than 70% of contribution rate. The main sources of NO3- in the water in the upstream and downstream of the cascade power stations were NH4+ in rainfall and fertilizer, as well as sewage and feces, which accounted for a similar proportion, totally contributing more than 80% of the sources of NO3-. In the Nyingchi section, the NH4+ in rainfall and fertilizer contributed the most to NO3-, accounting for 60%-70%, followed by the organic nitrogen in soil with 22%-28% of contribution rate. A large amount of farmland was distributed along the Yarlung Zangbo River and its tributaries. The nitrogen fertilizer applied in the farmland flowed into the river along with the surface runoff through rainfall, which was an important source of nitrogen in the water body of the Yarlung Zangbo River. Due to the reclamation of gentle slopes along the river and the erosion of cultivated soil, the organic nitrogen in soil was another important source of NO3- in the water of the Yarlung Zangbo River, especially in the wide valley section. Higher contribution rate of sewage and feces to NO3- in the canyon reach may be related to the development and construction of the cascade hydropower stations in the region. According to the change of DO contents and theoretical and measured values of δ18O-NO3-, nitrification and denitrification in the Yarlung Zangbo River are the main factors affecting the forms of nitrogen and the composition of nitrogen and oxygen isotopes. The results in the present study can provide data for the pollution control of nitrogenous compounds in the rivers in plateaus and drinking water security in Tibet.