Rapid, accurate, and dynamic diagnosis of nitrogen nutrition status is essential for evaluating vigor, for predicting production rates, and for agricultural management of cultivated wheat. To determine the critical nitrogen concentration dilution curve for wheat, two field experiments with different levels of N application (0, 75, 150, 225, 300 and 375 kg/hm²) were conducted in Yizheng, China. According to the procedure in Justes's curve, the dry matter and nitrogen concentrations under different nitrogen treatments could be compared by analysis of variance at the 5% probability level. Based on this curve, we developed a nitrogen nutrition index (NNI). In this index, when NNI=1, nitrogen nutrition was considered to be optimum, NNI>1 indicated excess nitrogen, and NNI<1 indicated nitrogen deficiency. We further investigated relationships between NNI and soil and plant analyzer development (SPAD) chlorophyll meter values and normalized differential SPAD_ij (NDSPAD_ij) values obtained at the top four leaf positions in wheat. We conducted a linear parallel curve analysis with grouped data to determine if the linear quantitative equation relating NNI to SPAD value or NDSPAD_ij showed differences between varieties and/or years. The results showed that the curve had specific biological significance. The relationship between the critical nitrogen concentration and wheat dry matter could be described by the following negative power equations Y16: N_cnc=4.65DM ^-0.44 and N13: N_cnc=4.33DM ^-0.45. The critical nitrogen curves of the two cultivars of wheat had the same coefficient "b" but different coefficients "a". Y16 had a higher nitrogen accumulation capacity than N13. The NNI values ranged from 0.37 to 1.28, and increased with higher nitrogen application rates under the different nitrogen levels. The optimum nitrogen level was 150 kg/hm² in 2009–2010, but was 225 kg/hm² in 2010-2011. The SPAD values increased with increasing N application rates, and ranged from 34.3 to 52.8. In contrast, the NDSPAD_ij value decreased with increasing nitrogen application rates, which showed that increasing amounts of nitrogen could decrease the differences in SPAD value among the top four single leaves of wheat. The NDSPAD_ij values ranged from 0.01 to 0.143. The SPAD value at the top four single leaf positions were significantly positively related to NNI. The correlation coefficients ranged from 0.666 to 0.823. The strongest correlation was between the SPAD value at the top leaf (T1) and NNI, while the weakest correlation was between the SPAD value at the second leaf (T2) and NNI. A linear parallel curve analysis with grouped data approximated the relationship between SPAD and NNI and its differences between varieties and years. This will be useful for diagnoses of wheat nitrogen nutrition. In contrast, NDSPAD_ij was significantly negatively related to NNI, except for NDSPAD₁₂. The correlation coefficients ranged from 0.01 to -0.849. The strongest correlation was between NDSPAD₁₄ and NNI, while the weakest correlation was between NDSPAD₁₂ and NNI. A linear parallel curve analysis with grouped data demonstrated the relationship between NDSPAD₁₄ and NNI (NNI=-2.019NDSPAD₁₄+1.18, R=-0.838^{* *}) stably across variety and year differences. This analysis could be used to assess NNI quantitatively and to diagnose wheat nitrogen deficiency quickly.