Abstract: This study aims to optimize the nitrogen management for cotton production under film-mulched drip irrigation with brackish water. A three-year nitrogen application levels experiment was conducted in Korla, Xinjiang, from 2017 to 2019. Xinluzhong cotton was taken as the test material. Nitrogen application levels were set at 0 (NF0), 150 (NF1), 250 (NF2), 300 (NF3), 350 (NF4), and 450 (NF5) kg/hm², where each treatment was received 487.5 mm of irrigation water. A systematic analysis was made to explore the effects of nitrogen application rate on the aboveground dry matter of cotton, nitrogen accumulation, yield, and nitrogen fertilizer use efficiency. A new model was constructed for the critical nitrogen dilution curve under film-mulched drip irrigation with brackish water. Results indicated that the aboveground dry matter increased with the nitrogen application rate early in the growing season. While there was the trend of increasing and then decreasing later stage. Nitrogen accumulation in cotton increased, as the growing season progressed. The critical nitrogen concentration was correlated with the maximum aboveground dry matter, following a power function. Both agronomic and apparent nitrogen use efficiencies showed a quadratic polynomial relationship with the nitrogen application rate. While the linear relationship was found in the physiological nitrogen use efficiency and partial factor productivity. These efficiencies increased with the decreasing nitrogen nutrition index at different growth stages. In the critical nitrogen concentration dilution curve model, parameters a and b were 3.967 and -0.227, respectively. The model was also fitted for cotton nitrogen concentration in 2018 and 2019. There was a linear relationship with the actual nitrogen concentration, with the root mean square error and standardized root mean square error of 0.085% and 3.645%, respectively. The nitrogen nutrition index indicated that the insufficient nitrogen supply was observed in the NF1, NF2, and NF3 treatments, thus limiting cotton growth. NF4 and NF5 shared the nitrogen nutrition index close to 1, indicating better nitrogen nutrition status. But NF5 treatment presented the lower yield and nitrogen use efficiency, compared with NF4. The highest yield and nitrogen use efficiency were achieved in the NF4 treatment. Therefore, the nitrogen application rate of 350 kg/hm² was recommended for cotton under film-mulched drip irrigation with brackish water in southern Xinjiang. The dilution curve of critical nitrogen concentration and nitrogen nutrition index can be expected to diagnose the nitrogen nutrition for the optimal nitrogen application in cotton field.