Abstract: Water and nitrogen have great influences on the production of tomato in greenhouses. This study aims to determine the applicability of the critical nitrogen concentration dilution curve model in different water conditions, and to suggest a rapid and accurate method of identifying the nitrogen condition of tomato. Greenhouse experiments with different water-nitrogen combinations were conducted in 2013-2015 in Yangling, Shaanxi. Four water treatments were investigated, i.e., full irrigation through the whole life span of tomato, deficit of 50% at seedling stage, deficit of 50% at flowering stage, and deficit of 50% through the whole life span. Three nitrogen treatments were investigated, i.e., 0, 150, and 300 kg/hm2. We validated and verified the critical nitrogen concentration dilution curve model with the data in 2013-2015. We verified the applicability of the model under different water treatments of greenhouse tomato in Northwest China. We built relationship between the model parameters and the average daily water consumption of tomato during the whole growth stage, and improved the applicability of the critical nitrogen concentration dilution curve model under different water treatments. Both the parameter a and the parameter b of the critical nitrogen concentration dilution curve model decreased first and then increased with the increase of the average daily water consumption during the whole growth period of the tomato. We analyzed the relationship between SPAD (soil and plant analyzer development) values and nitrogen nutrition index (NNI) at different leaf positions with the test data in 2013-2015. There was a linear correlation between SPAD and NNI at different leaf positions of tomato under different water treatments. The fitting relationship between the SPAD values and the NNI of the upper and lower leaves was poor, and the degree of fitting was greatly affected by the year and the treatment. The fitting relationship between the median leaf's SPAD value and NNI was a significant linear positive correlation relationship and had good stability. Therefore, the tomato median leaf can be used as a suitable diagnostic leaf for nitrogen diagnosis. The results showed that: 1) The model could well estimate the critical nitrogen concentration of tomato with the absolute error of 0.13-0.34 g/ 100 g, the standard error of 0.14-0.39 g/ 100 g, and the decisive factor (R2) of 0.94-0.99. 2) At the median position of canopy, there was a good linear correlation between the SPAD value and NNI, and the SAPD values of the middle leaves can be used to determine the nitrogen conditions of tomato. 3) The SPAD values of middle leaves at the NNI of 1 can be the indicators for appropriate nitrogen fertilizing. When the SPAD value of the median leaf in the different treatments is greater than the appropriate SPAD value, it indicates that the application of nitrogen in the treatment is excessive and should be appropriately reduced. When the SPAD value of the medial leaf in the test treatment is less than the appropriate SPAD value, it indicates that the application of nitrogen should be appropriately increased. Therefore, we have determined and validated the applicability of the critical nitrogen concentration dilution curve model under different water treatments of greenhouse tomato in Northwest China and constructed a nitrogen nutrition diagnosis method based on the daily water consumption of tomato, which can support nitrogen diagnosis and nitrogen management of tomato in greenhouses in this region.