Abstract: To clarify the regulatory mechanism of regulated deficit irrigation(RDI)regimes at different growth stages on the yield and nutritional quality of solar greenhouse pepper in the arid northwest region of China, a single-factor randomized block design was employed. Two irrigation levels—30% field capacity(FC)and 60% FC—were set at three critical growth stages(flowering stage, early harvest stage, and late harvest stage), with conventional full irrigation throughout the entire growth period(CK, typically 90% FC)as the control. The effects of water deficit at different growth stages on yield formation, nutritional quality indicators, and water use efficiency(WUE)of pepper were systematically analyzed. The results showed that water deficit at different growth stages had significant impacts on both yield and nutritional quality of pepper, and the optimal water management combination balancing yield stability and quality improvement exhibited growth-stage specificity. Sensitivity analysis revealed that the priority order of irrigation level impacts on core pepper indicators was: vitamin C(Vc)content > yield > soluble solids content > fruit moisture content > fruit firmness. Implementing 30% FC RDI from the transplanting to flowering and fruit-setting stages of greenhouse pepper significantly improved WUE and effectively enhanced the internal nutritional quality of fruits compared with conventional full irrigation.