The interannual stability and adaptability of the critical nitrogen dilution curve (LNCc) of drip-irrigated cotton in alkaline soil in northwest inland region (medium-ripe cotton region in Southern Xinjiang) were verified based on leaf biomass (LBM) . The consistency of nitrogen nutrition status and the dosage of nitrogen fertilizer by leaf nitrogen nutrition index (LNNI) model，the relationship model of leaf nitrogen concentration，nitrogen levels (Nlevel) and seed cotton yield(Y) for alkaline soil drip-irrigated cotton in northwest inland region (medium-ripe cotton region in Southern Xinjiang) were clarified. This study was conducted based on two-year field experiment with five nitrogen levels (0，75，150，300，450 kg/hm²) and two cotton cultivars (Xinluzhong55，Xinluzhong78). LNCc，LNNI and the relationship model of leaf nitrogen concentration and nitrogen levels were established based on LBM of different cotton varieties. The results showed that:(1) The relationship between LNC and LBM could be described by the power equation，and the model described as two parts (for Xinluzhong55:if LBM≥1.02 t/hm²，LNCc=3.41×LBM-0.38，while if LBM<1.02 t/hm²，LNCc=2.18%；for Xinluzhong78:if LBM≥1.07 t/hm²，LNCc=4.08×LBM-0.43，while if LBM<1.07 t/hm²，LNCc=2.61%). (2) Model validation with independent crop growth data showed that the LNCc of alkaline soil drip-irrigated cotton was simulated satisfactorily. Root mean square error (RMSE) and modelling efficiency indexes (ME) of two cotton cultivars (Xinluzhong55，Xinluzhong78) simulated and observed LNCc based on the 1∶1 line were 0.941，0.116 g/100 g，0.944 and 0.975，0.088 g/100 g，0.975，respectively，with fine stability and adaptability among years. (3) The LNNI of Xinluzhong55，Xinluzhong78 ranged from 0.76～1.12，0.70～1.09 with different years and nitrogen levels，respectively，and the LNNI model resulted in the suitable levels of N 300 kg/hm² for both Xinluzhong 55 and 78. (4) Based on the relational model of LNC and Nlevel and Y，the LNC and Y during the boll setting period (BS) and boll opening period (OS) were 2.32%，1.95%，5850，5860 kg/hm²，and 2.30%，1.99%，6070，5970 kg/hm²，respectively. The suitable nitrogen application range was between 313.3~380.0 and 335.0~385.0 kg/hm². This was basically consistent with the evaluation results of the appropriate nitrogen application rates for Xinluzhong 55 and 78 using the nitrogen level and seed cotton yield relationship model. The results can provide theoretical basis and method reference for accurate diagnosis of nitrogen status and appropriate nitrogen application evaluation in alkaline soil with drip-irrigated cotton in Northwest inland region (medium-ripe cotton region in Southern Xinjiang).