The critical nitrogen (N) dilution curve (CNDC) describes the process of achieving the highest yield with the lowest nutrient concentration. It is influenced by species and environment, providing a new strategy for diagnosing cotton nitrogen status and its impact on cotton growth and yield. This study investigated the effects of two irrigation amounts (45 and 54 mm) and three N application rates (150, 225, and 300 kg ha⁻¹). Results indicated that compared with low water treatment (W1), increasing irrigation water led to a 21.3–24.7% increase in dry matter, a 22.2–29.6 % increase in N accumulation, a 1.4–26.1 % decrease in soil N content, and a 3.9–9.0 % reduction in N percentage of reproductive organs. The nutrient network index (NNI) decreased by 2.2–6.4 %, while irrigation water productivity (WP_I) decreased by 5.4–22.1 %. Compared with low nitrogen (F1), increasing N fertilization application enhanced dry matter by 7.1–20.8 % and N accumulation by 2.7–24.8 %, while also increasing the nitrogen percentage in reproductive organs by 4.0 –9.9 %. Experimental results demonstrated that moderate increases in irrigation and nitrogen application improved seed cotton yield and the partial factor productivity of N (PFP_N). Under the W1 treatment, higher nitrogen application brought cotton plant N closer to a optimal condition. However, excessive water and N exacerbate N loss and result in a nutrient network index (NNI) value below 1, indicating prolonged nutrient deficiency. The TOPSIS comprehensive evaluation indicates that an irrigation water amount of 45 mm and a nitrogen application rate of 300 kg ha⁻¹ represent optimal water and nitrogen application strategies under water resource constraints. Conversely, an irrigation water amount of 54 mm and a nitrogen application rate of 225 kg ha⁻¹ are identified as the optimal strategies for achieving high cotton yields.