Abstract：Nitrogen is a main nutrient element for crop growth but the appropriate nitrogen application and management is important in the Yellow River Irrigation Area of Ningxia. So, an effective method of accurate nitrogen assessment for maize is urgently needed in this area. In this study, based on leaf area index (LAI) of maize under drip-irrigated system, a critical nitrogen dilution curve model was established and its applicability for nitrogen management was assessed. The feasibility of diagnosing nitrogen nutrition status of maize under a water and fertilizer drip-integrated mode by using nitrogen nutrition index (NNI) was also evaluated. Six nitrogen application rates with three replicates each were designed including 0 (N0)、90 (N90)、180 (N180)、270 (N270)、360 (N360), and 450 (N450) kg/hm2 in 2017 and 2018. The LAI, dry matter and nitrogen concentration were determined at jointing stage, trumpet stage, large trumpet stage, tasseling stage, and silking stage, respectively. Then, the quantitative relationship between LAI and plant nitrogen concentration (PNC) were analyzed. Moreover, the critical nitrogen dilution curve model based on LAI was constructed and verified after analysis of aboveground biomass accumulation and plant nitrogen concentration of maize from jointing stage to silking stage under different nitrogen application rates. The critical nitrogen dilution curve model was employed to reveal the effects of nitrogen application rates on NNI of maize at different growth stages. A theoretical framework was established to reveal the relationship between LAI and plant dry matter (PDM) curves and to compare the differences between critical nitrogen curve based on LAI and that based on PDM. The results showed that, as a whole, both LAI and plant nitrogen concentration were increased as nitrogen application rate increased. However, they increased with two distinct periods, which could be classified into nitrogen limitation and nitrogen non-limitation periods. The relationship between critical nitrogen and LAI conformed to a power function model at the vegetative growth stage of maize. Meanwhile, the root mean square error (RMSE) and normalized root mean square error (n-RMSE) of this established model were 0.09 and 4.13%, respectively. It indicated this model presented a good stability to predict critical nitrogen concentration of maize as well as to evaluate plant nitrogen nutrient at the vegetative growth stage. Furthermore, the NNI increased from 0.53 to 1.34 when the nitrogen fertilizer application rate increased at different growth stages. Hence, NNI could accurately reflect the nitrogen nutrition status of maize, and 270 kg / hm2 was the best application rate of nitrogen in this study. In addition, under non-limited nitrogen condition, the nitrogen absorption of maize plants presented a positive correlation with LAI and the allometric growth parameters of LAI and PDM were closer to two thirds of the theoretical value. In conclusion, the established critical nitrogen curve based on LAI can effectively identify the nitrogen condition required by the plant from the jointing stage to the silking stage. The critical nitrogen curve based on leaf area index provides an effective evaluation method for precise nitrogen management of maize under drip-irrigated system in Ningxia.