Context or problem: Mung bean (Vigna radiata L.), a key crop for nutritional security and ecological restoration in semi-arid regions, suffers from yield stagnation and suboptimal fertilization efficiency. Despite its agronomic importance, the synergistic mechanisms by which nitrogen (N) and phosphorus (P) regulate photosynthetic carbon assimilation, source-sink dynamics, and grain quality formation remain poorly understood. Objective: To address this knowledge gap, we conducted a two-year field experiment (2022-2024) employing a complete factorial design with four N (0, 60, 90, 120 kg ha(-1)) and P (0, 45, 90, 135 kg ha(-1)) application rates (16 treatments total), using the widely cultivated Yulv 1 variety. The research systematically evaluated N-P coordination effects on photosynthetic performance, dry matter accumulation, yield components, and grain quality in mung beans. Results: Our results demonstrate that synergistic N-P co-application significantly enhanced leaf nutrient content, SPAD values, and photosynthetic efficiency, thereby delaying functional leaf senescence during reproductive growth. The N-P interaction optimized dry matter partitioning, elevating pod allocation by 27.36 % and boosting grain yield by 41.92 % compared to unfertilized controls. Mechanistically, N dominated chlorophyll biosynthesis via leaf N modulation, indirectly influencing carbon allocation (grain number), while P regulated grain morphogenesis (100-grain weight) through P-mediated pathways. Notably, excessive fertilization (> 90 kg ha(-1)) induced photoinhibition, disrupted nutrient-defense metabolic imbalance (evidenced by an 18.7 % reduction in flavonoid content), and led to grain deformities, highlighting a trade-off between yield maximization and physiological stability. Structural equation modeling identified leaf nutrient and SPAD values as the central hub linking photosynthetic performance, biomass accumulation, and yield architecture. Furthermore, fertilization-induced variations in mung bean grain color were significantly correlated with shifts in nutrient composition, suggesting a physiological link between fertilization regimes and grain quality traits. Conclusions: In summary, Optimal N-P co-application (90 kg ha(-1) each) synergistically enhanced photosynthetic efficiency, dry matter allocation to grains, and yield in mung beans while maintaining a balance between nutritional and appearance quality. Implications or significance: This work establishes a physiologically grounded framework for reconciling yield enhancement with nutritional enrichment in rainfed legumes, offering actionable strategies to transform semi-arid pulse production systems through precision nutrient stewardship.