Large rainfall fluctuation and excessive nitrogen application are the main factors that limit the nitrogen fertilizer efficient utilization and stable yield and high quality in winter wheat production in Weibei dryland. In this study, the purpose was to construct different critical nitrogen concentration dilution curves of winter wheat under two precipitation patterns to analyze the feasibility of diagnosing the nitrogen nutrion status of dryland winter wheat by the nitrogen nutrient index. From 2017 to 2021, a 4-year nitrogen application test was conducted in Heyang County, Shaanxi Province, with Jinmai 47 as the test material. Five nitrogen application rate treatments were set at 0, 60, 120, 180 and 240 kg/hm². According to the classification of precipitation patterns, normal years were 2017-2018 and 2020-2021 , and dry years were 2018-2019 and 2019-2020. The effects of different nitrogen application rate on nitrogen utilization efficiency, grain yield and its components of winter wheat under the two precipitation patterns were srudied. Based on the relationship between the aboveground biomass and plant nitrogen concentration of winter wheat in dryland under the two precipitation patterns, the critical nitrogen concentration dilution curve model and nitrogen nutrient index (NNI) was established. The results show that: 1) nitrogen application, precipitation patterns and their interaction effects have a significant impact on the grain number per spike, 1000-grain weight and grain yield. 2) The aboveground biomass and critical plant nitrogen concentration of winter wheat under normal years and dry years were conform to the power function relationship. However, there are variances in models parameters of normal years and dry years(the model parameter a were 3.33 and 2.79 g/kg in normal and dry years, and the parameter b was 0.40 and 0.31 in normal and dry years, respectively). The model validation showed that the plant nitrogen concentration fitted by the critical nitrogen concentration dilution curve model was linearly correlated with the actual nitrogen concentration. The root mean square error (RMSE) was 0.20 and 0.14 g/kg in normal years and dry years, respectively, and the normalized root mean square error (NRMSE) was 10.30% and 7.69%, respectively, indicating that models had good stability. 3) According to the grain yield and nitrogen nutrition index, the appropriate nitrogen rates in normal and dry years was 160-180 kg/hm² and 101-120 kg/hm². In this study, the critical nitrogen concentration dilution curve and nitrogen nutrient index were established based on the aboveground biomass and plant nitrogen concentration of dryland winter wheat during each growth states under the two precipitation patterns. The critical nitrogen concentration dilution curve and nitrogen nutrient index better predicted the nitrogen nutrient status of winter wheat in different growth stages under the two precipitation patterns. The results can provide valuable information for the nitrogen assessment and precise nitrogen application.