Abstract: Abstract: Guanzhong plain of Shaanxi Province is one of the main productions of summer maize, and faces 2 problems: 1) Annual and seasonal large rainfall variability can significantly affect dry matter accumulation of summer maize and growth; and 2) Nitrogen fertilizer is excessively applied because the increase of nitrogen rate application does not synchronize with the increase of maize yield, but lead to a series of environmental problems. The existing crop nitrogen diagnostic tool is time-consuming and costly, so exploring simple and practical techniques for scientific nitrogen application is necessary such as nitrogen dilution curve. In this study, we constructed nitrogen dilution curves and validated its feasibility to diagnose nutrient status in Guanzhong plain. Six summer maize varieties and four nitrogen levels N0(0), N1(86 kg/hm2), N2(172 kg/hm2) and N3(258 kg/hm2) were applied to the study. The study was carried out in College of Water Resources and Architectural Engineering, Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas of Ministry of Education Northwest A&F University, Yangling, Shaanxi China (108°24′E, 34°20′N) in 2013 and 2014. These two years were different with normal rainfall at early stage and drought at late stage in 2013 but drought in early stage and flooding in late stage in 2014. Aboveground biomass, nitrogen content and yield of summer maize were determined. Based on dry matter, the critical nitrogen dilution curves for summer maize were developed and validated. Results showed that exponential function with two parameters including critical N concentration for 1 ton dry matter (CC) and changes of critical N concentration (CR) could be used to describe relationships between the nitrogen concentration and the aboveground biomass in 2 years. In 2013, CC values were 21.91 and 22.3 for dry matter larger and lower than 1.5 t/hm2; in 2014, CC values were 21.91 and 26.5 for dry matter larger and lower than 0.63 t/hm2, respectively; The CR values were 0.31 and 0.14 for the year of 2013 and 2014, respectively. The models were validated using data of another maize variety. The standardized root mean square error was 8.8% and 14.1% for the year of 2013 and 2014, indicating that the model fitness was well. The model parameter CC was not affected by years with different rainfall conditions. However, the parameter CR was greatly affected by years in that the value was higher in 2013 than 2014. Based on the critical N dilution curves, nitrogen nutrition index (NNI) was calculated to quantify the nitrogen nutrition status in main growth period of summer maize. The diagnostic results showed that NNI increased with N application rate and it was higher in 2014 than in 2013. The NNI was less than 1 for N0 treatment, indicating N deficiencies, but greater than 1 in N3 treatment, indicating excess N application. By analyzing relationship between NNI and relative yield, it was found that the optimal N application rate was 172 kg/hm2 in 2013. The results can be used for the diagnosis of N nutrition in maize. It also can provide a key technical approach to precise N fertilization management in summer maize production in Guanzhong Plain.