Abstract: To provide a theoretical reference to nitrogen fertilizer management for maize and eco-enviromental protection in the irrigated area of Ningxia, we established the critical nitrogen concentration (N_c) dilution curve of maize under drip-irrigated condition. The nitrogen nutrition index (NNI) was deduced from the established N_c curve model, to diagnose feasibility of nitrogen nutrition. Six levels of nitrogen application rates including 0, 90, 180, 270, 360 and 450 kg·hm^-2 were conducted in field with maize variety ‘Tianci 19’. During two-year field study, the N_c dilution curve and NNI model of maize were established based on leaf dry matter (LDM). The results showed that: 1) There was a negative power function relationship between LDM and N_c, which presented as two parts, if LDM<1.15 t·hm^-2, N_c=3.2%, while if LDM≥1.15 t·hm^-2, N_c=3.29LDM^-0.29. 2) The root mean square error (RMSE) and standardized root mean square error (n-RMSE) of the model were 0.203 and 8.0%, respectively, with fine stability among years. 3) The NNI ranged from 0.47 to 1.44 with different nitrogen application rates. In addition, NNI was positively correlated with yield and negatively correlated with agronomic utilization efficiency of nitrogen fertilizer at different growth stages of maize. Therefore, NNI could be used to explain maize yield changes with and without nitrogen limit under drip-irrigated system. In conclusion, the critical nitrogen dilution curve established from leaf dry matter could be applied to accurately estimate nitrogen nutrition status throughout the whole growing stages from the jointing stage to silking stage of maize under drip irrigated system.