Nitrogen (N) application can improve drought tolerance and water use efficiency (WUE) in crops. Previous studies have shown that aerated irrigation improves crop N absorption and utilization. However, the mechanisms behind the interaction of water and N under aerated drip irrigation and its impact on crop WUE remain unclear. This study conducted a two-year greenhouse experiment with spring-summer and autumn-winter tomato crops to investigate the effects of water and nitrogen coupling on leaf carbon (C) and N content, photosynthetic characteristics, dry matter accumulation, yield, and WUE. The experiment included three irrigation levels (W1, 50% ET _c; W2, 75% ET _c; W3, 100% ET _c) and three N application rates (N1, 0 kg ha ^–1; N2, 150 kg ha ^–1; N3, 250 kg ha ^–1). The results showed that increased N application and irrigation levels significantly increased leaf C and N content, net photosynthetic rate (P _n), and stomatal conductance (G _s) ( P < 0.05). Under deficit irrigation, N application increased leaf C content by 2.17% and N content by 9.34%, improving leaf photosynthetic capacity and increasing P _n by 15.57% and G _s by 19.32%. The W2 treatment demonstrated the most pronounced improvements compared to W1. The W3N3 treatment produced the highest dry matter accumulation for both tomato types, with no significant difference from W2N3 ( P > 0.05). The W2N3 treatment produced the highest yield, 8.67–9.13% higher than W3N3. The highest WUE occurred in W2N3 for spring–summer tomato and W1N3 for autumn–winter tomato. Although W1N3 had 1.02% higher WUE than W2N3, it had a 15.25% lower yield. Thus, W2N3 is recommended as the optimal water–nitrogen management strategy for greenhouse tomato production. Correlation analysis revealed that leaf C and N contents positively correlated with P _n, dry matter accumulation, and yield, while the leaf ratio of C and N (C/N) negatively correlated with WUE, suggesting that leaf C and N contents regulate tomato WUE. N application under deficit irrigation enhanced leaf C and N contents, improving photosynthetic capacity (P _n, G _s), dry matter accumulation, yield, and WUE. Regression models suggest that the optimal water and N application rates for greenhouse tomatoes are 192.30–225.67 mm and 205.93–243.43 kg ha ^-1 for spring-summer tomato, and 162.00–181.18 mm and 194.98–237.73 kg ha ^-1 and for autumn-winter tomato crops. These findings provide a theoretical basis for water-efficient agricultural practices and sustainable greenhouse tomato production.