Xinjiang is one of the most prolific tomato-planting areas in China. Here, we carried out a two-year (2017–2018) field experiment in Xinjiang to study the effects of different nitrogen (N) application rates on the spatial distribution of water and salt in the root zone, as well as their impacts on the yield and quality of tomatoes under mulched drip irrigation. The ideal ranges of N application rates for tomato yield and quality were examined under different salinity levels. Results indicated that soil water content and salinity increased with soil depth. Soil water content was closely related to soil salinity but not to N. Among the tested application rates, tomato yield was highest under the medium-high N (225–300 kg/ha) and low salt (4 g/kg) treatment. Under the highest salt level (10 g/kg), the low nitrogen treatment (150 kg/ha) was better than the high N treatment (300 kg/ha) at boosting tomato yield. Moreover, we found that salinity had a stronger effect on tomato quality than N. Based on these results, we were able to recommend ideal ranges for N (155–201 kg/ha) and salt (3.56–5.59 g/kg) while both are present in the soil.