<p id="C3">Sugar beet is one of the two major sugar crops in China. However, both nitrogen sufficiency and deficiency will affect the yield and sugar content in sugar beet, and excessive nitrogen will cause environmental risks. Judging nitrogen nutrition of sugar beet in a reasonable range is of great significance for ensuring the yield, quality and reducing environmental risks of sugar beet. The critical nitrogen concentration dilution curve is the basis of crop nitrogen nutrition diagnosis. The objective of this present study was establishing the dilution curve model of the critical nitrogen concentration of sugar beet in China and to diagnose the nitrogen nutrition of sugar beet by using the corresponding nitrogen nutrient index. Four experiments with different N rates for two cultivars of sugar beet were conducted in Inner Mongolia from 2017 to 2018. The above-ground biomass and under-ground tubers were sampled and their nitrogen concentration was determined at seedling, leaf cluster growth, tuber bulking, sugar accumulation, and harvest stages. The total plant biomass and nitrogen concentration were calculated. Based on the calculation, the critical nitrogen concentration dilution model and the corresponding nitrogen nutrient index were established. The results indicated that the negative power relationship between the above-ground biomass and nitrogen concentration was observed. The determination coefficient (<italic>R</italic> <sup>2</sup>) for the relationship was 0.45 on average. The <italic>R</italic> <sup>2</sup> for the negative power relationship between total plant biomass and nitrogen concentration was 0.94 on average, which was higher than the relationship between the above-ground biomass and nitrogen concentration. It was more reasonable to establish a critical nitrogen concentration dilution model based on the total plant biomass and nitrogen concentration, which was less affected by the cultivars. The <italic>R</italic> <sup>2</sup> for the integrated critical nitrogen concentration dilution curve of sugar beet varieties KWS9167 and KWS1676 reached 0.94. The constructed nitrogen nutrition model can be used to estimate the plant nitrogen status in sugar beet. The critical nitrogen concentration dilution model for the main sugar beet varieties in Northern China was <italic>N</italic><sub>c</sub> = 4.23<italic>W</italic> <sup>-0.49</sup>. The optimized nitrogen rate of KWS9167 and KWS1676 based on the model was about 160 kg hm <sup>-2</sup> and 180 kg hm <sup>-2</sup>, respectively.