<p>The construction and operation of large reservoirs alter the natural hydrological regimes of rivers, yet their impacts on the migration and transformation of nutrients have not fully been understood. In this study, the Jingjiang Reach in the middle-stream of Yangtze River was selected to investigate the effect of the operation of the Three Gorges Reservoir (TGR) on the migration and transformation of nitrogen (N). Long-term hydrological and water quality monitoring data (from 1992 to 2015) for Yichang, Zhicheng, Shashi, and Chenglingji along the Jingjiang Reach were analyzed, and the distribution characteristics of N in overlying water were quantified. The results shows that the annual average total nitrogen (TN) concentration in the Jingjiang Reach increased from 0.97 mg/L in 1992 to 1.94 mg/L in 2015, while the ammonium nitrogen (NH₄⁺-N) concentrations showed a downward trend. The variation trends of both N forms shifted around 2003 when TGR begins to storage water. Seasonal variations were observed, with the highest concentrations of TN and nitrate nitrogen (NO₃⁻-N) occurring during the normal water period (March–May). During this period, the average TN concentration across the four monitoring sites was 33% and 19.8% higher than those during the dry and wet periods, respectively. In contrast, the peak concentrations of NH₄⁺-N was observed during the dry period (October-February). Additionally, correlation analysis and Redundancy Analysis (RDA) revealed that Chemical Oxygen Demand (COD) and the distance from the TGR were the most influential factors on N concentrations in the Jingjiang Reach, with corresponding contribution rates of 36.8% and 32.5%. The distance from TGR exerted a significant positive effect on NH₄⁺-N concentration, while COD showed a significant negative effect on TN concentration. The results suggested that the impoundment of the TGR posed a significant impact on the N content in the Jingjiang Reach, providing a theoretical support for nutrient management in the Yangtze River.</p>