Abstract: The unreasonable application of water and nitrogen significantly affects the growth and the yield of greenhouse vegetables in northwest China, which has raised the need to adjust the crop irrigation and nitrogen (N) fertilizer requirements. Based on the aboveground biomass, the assessment of crop N status is an effective approach to optimize N management in greenhouse vegetable production. The research was conducted in a sunlit, ventilated greenhouse at the Key Laboratory of Agricultural Soil and Water Engineering in Arid Areas, Ministry of Education, in Yangling District, Shanxi Province, during the spring-summer (April-July) seasons of 2014 and 2015. The site is located at 34°20′N, 108°04′E. There were 4 irrigation levels (W1: 105%ET0, W2: 90%ET0, W3: 75%ET0, and W4: 60%ET0, ET0 is reference crop evapotranspiration) and four nitrogen levels (N1: 300, N2: 225, N3: 150, and N4: 75 kg/hm2), resulting in a total of 16 treatments. The plant samples at different treatments were taken 8 times during the study periods, i.e.,33, 54, 62, 72, 81, 91, 101, and 112 days separately after being transplanted, which were used to measure the aboveground biomass and total nitrogen of bell peppers. The yields of bell pepper were measured during the harvesting time. Soil moisture was measured before transplanting and after harvest using the time domain reflectometry. Water use efficiency was calculated by water balance method. Results showed that the total N uptake, aboveground biomass, yield and water use efficiency (WUE) were affected by the irrigation levels. The diagnosis results of the greenhouse pepper's nitrogen nutrition based on critical N model were the same between the model of nitrogen nutrition (NNI) and the model of N uptake. The optimal N fertilizer were between 150 and 225 kg/hm-2, but more closer to 225 kg/hm-2. The irrigation levels of 75%ET0 and 90%ET0 were more favorable to the increase in nitrogen nutrition uptake. The 2 irrigation levels economic yields were 34.14 and 35.33 t/hm2, and WUE were 17.07 and 15.30 kg/m3, respectively. However, the economic yield and WUE could not achieve optimal results at the same time. The aboveground biomasses of bell pepper at the 2 irrigation had no significant difference when the N application rate was more than 150 kg/hm2. The optimal N application rates were 194.00 and 192.69 kg/hm2 based on the economic yield, while the optimal N application rates were 191.13 and 191.83 kg/hm2 based on WUE. Considering the water shortage condition in the northwest China, the irrigation level of 75%ET0 (N fertilizer application rate of about 190 kg/hm2) was suggested to be optimal for greenhouse bell pepper production since it could save water by about 11%, while the economic yield only declined by about 3% compared with the irrigation level of 90%ET0. The study can provide theoretical basis and technical support for improving water and nitrogen management of greenhouse bell peppers in northwest China.