Abstract: The risk of river eutrophication significantly increased in Qin River Basin with increasing nitrate by anthropogenic activities. In order to identify nitrate sources in the upper reaches of Qin River, twenty-eight surface water samples were collected in October 2017, March 2018 and June 2018. The combined use of stable isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-, δD-H₂O and δ¹⁸O-H₂O) and water chemistry parameters (Cl^-, NO₃^--N, NH₄⁺-N) were used to evaluate nitrate sources and transformation processes. The fractional contributions of potential nitrate sources were calculated using a Bayesian mixture model. The results were as follows: (1) Nitrate is the dominant form of dissolved inorganic nitrogen. The average concentration of NO₃^--N in autumn ((2.40±1.17) mg/L) was significantly higher than that in spring ((2.11±1.03) mg/L) and summer ((1.50±0.61) mg/L). δ¹⁸O-NO₃^- (13.0‰±3.13‰) values in spring were significantly higher than that in summer (2.90‰±3.12‰) and autumn (6.62‰±1.30‰). (2) Nitrate is mainly from soil nitrogen and chemical fertilizer. According to the results of SIAR stable isotopes model, the fractional contributions from soil nitrogen in spring, summer, and autumn were 27.8%, 39.5% and 39.3% respectively, from fertilizer were 29.1%, 40.2% and 41.9%, respectively. The fractional contribution from atmospheric deposition in spring (24.7%) was significantly higher than that in summer (2.4%) and autumn (10.6%). (3) Nitrate in the upper reaches of Qin River is mainly affected by the nitrification process, and is notinfluenced by denitrification process. The research showed that nitrate in the upper reaches of Qin River is mainly from nitrification of soil organic nitrogen and ammonia in chemical fertilizers. Measures for reducing NO₃^- inputs to rivers should be considered by improving effectively utilizing rate of chemical fertilizer and inhibit nitrification.