The critical nitrogen concentration in plant was defined as the minimum nitrogen concentration required for maximum plant above-ground biomass. By collecting and analyzing seven-site-year field experiments with various nitrogen application rates according to certain criteria, the critical nitrogen concentration dilution curve was constructed for the main winter wheat variety “Xiaoyan 22” in Guanzhong Plain. The relationship between maximum above-ground biomass and critical nitrogen concentration of winter wheat could be described by a negative power function. The nitrogen dilution curves affected by different nitrogen treatments were evaluated and compared. Results showed that increased nitrogen supply generally resulted in a greater nitrogen concentration. The winter wheat yield with zero, deficient, suitable and excess nitrogen were 4 263, 5 097, 6 023 and 6 203 kg/hm2, respectively. Nitrogen deficiency would reduce wheat yield, however, there was no significant difference between the yield of excessive and suitable nitrogen application treatments. The dilution curve was validated with independent experiment data (2010—2011) through nitrogen nutrition index (NNI), which was defined as the ratio of measured to critical nitrogen concentrations. The results showed that the NNI value with zero, deficit, suitable and excess nitrogen application treatments were 0.70, 0.89, 1.01 and 1.25, respectively. Wheat would get nutrition imbalance when nitrogen was used only as a base fertilizer and even reduce crop yield with excess nitrogen application. The suitable nitrogen application rate was 75~180 kg/hm2 (averagely 137 kg/hm2) for winter wheat in Guanzhong Plain due to the variety of soil productivity. The critical nitrogen dilution curve could be used as a diagnostic tool to detect wheat nitrogen nutrition, and it also provides a key technical approach for guiding nitrogen fertilization practices and management in wheat production in Guanzhong Plain.