The crop critical nitrogen concentration is the plant minimum nitrogen concentration for the crop to reach the maximum growth. The critical nitrogen concentration decreases with the increase of the dry matter accumulation of the above-ground plants，which can be expressed by the power function model of Nc=a×Wmax-b. Critical nitrogen dilution models for dozens of crops have been established. According to the characteristics of crop harvest organs，they are divided into four categories:harvesting grains，harvesting fresh fruits，harvesting roots and tubers，and harvesting aboveground stems and leaves. Different types of critical nitrogen concentration dilution models have their own characteristics in terms of the parameters and nitrogen diagnostic applications. In addition to the typical form of the critical nitrogen concentration of the plant-the dry weight of the plant，the critical nitrogen concentration dilution model also has the critical nitrogen concentration of the organ-the dry weight of the organ，the critical nitrogen concentration of the plant-the quantitative index of the organ(leaf dry weight，stem and sheath dry weight，LAI，leaf area duration)and other forms. The basic principles for selecting the type of critical nitrogen concentration dilution models are proposed. Because the growth characteristics，nitrogen nutrition characteristics and dry matter productivity of different crop types，and even different varieties of the same crop under different climatic conditions are greatly different，it is difficult to establish a general model for a certain category of crops or even a certain crop under various climatic regions and therefore a certain type of crop with similar characteristics in a certain climate region and certain cultivation mode should be used as the basic unit for building the models. The applications of crop critical nitrogen concentration dilution model include the calculation of nitrogen nutrition index(NNI)for nitrogen nutrition diagnosis，calculation of critical nitrogen accumulation，deficit and nitrogen fertilizer requirements，and prediction of crop yield and quality. The limitations of the nitrogen dilution model driven by plant dry matter are pointed out that the increase in the maximum plant dry matter weight is essentially a measure of the process of crop development. The concept of critical nitrogen concentration provides a scientific method to determine the appropriate nitrogen concentration at a certain growth stage. A research technical route of “the climate zoning，cultivation method selection and varieties classification-determination of the critical nitrogen concentration at different growth stages through experiments-establishment of a dynamic model of critical nitrogen concentration driven by growth process” is proposed.