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A critical nitrogen concentration in plant above-ground biomass, which is defined as the minimum nitrogen concentration required for maximum plant growth, can be found at any time in the growth cycle. To determine the critical nitrogen (N) concentration dilution curve with different nitrogen levels for drip-irrigation cotton in the Manas County of Xinjiang. Field experiments were carried out with cultivars of Xinluzao 45 and five nitrogen rates (0kg/hm2, 120kg/hm2, 240kg/hm2, 360kg/hm2, and 480kg/hm2) were applied. Results showed that N concentration in above-ground biomass was declined with accumulated physiological development time after emergence. The relationship between the above-ground biomass and critical N concentration can be described by the power equation (Nc=3.91W-0.24max), with a=3.91 and b=0.24. The relationship between the correlation coefficient was 0.906. Through the root mean square error (RMSE) to verify this model, validation results showed that the model had a good simulation performance. Based on the critical N concentration model, the model of nitrogen nutrition index (NNI) was developed. According to the NNI under critical N concentrations, it was concluded that 240~360kg/hm2 could be used as the optimum N application rate of drip-irrigation cotton in Xinjiang.