Citrus yield and fruit quality are severely limited by inefficient water and fertilizer management in Southwest China. Formulating appropriate deficit irrigation and fertilization (DIF) schedules is necessary for achieving sustainable production of citrus. Herein, three irrigation levels (CK, 100% control treatment; I80, 80% CK; and I60, 60% CK) and four fertilization levels (CK; F85, 85% CK; F70, 70% CK; and F55, 55% CK) were used at each growth stage of citrus. Multiple regression models efficiently predicted the effects of DIF on citrus water productivity (WPc), partial fertilizer productivity (PFP), fruit quality, and yield, with an R2= 0.71-0.98**. Specifically, the application of more fertilizer during stage III improved WPc, soluble sugar (SS) content, and vitamin C (Vc) content; these indices peaked when the irrigation amount was 280, 322, and 306 mm, respectively, and the fertilizer (N/P2O5/K2O) application was 483/480/762 kg/ha during stage III. SS and Vc contents increased as irrigation amount decreased and fertilization application rate increased during stage IV. During stage IV, a decrease in irrigation amount significantly reduced titratable acid content. Finally, different parameters were optimized using regression models and spatial projection methods. During stages I, II, III and IV, the optimal irrigation amounts were 28-36, 74-93, 304-334, 151-197 mm respectively, and the optimal fertilizer (N/P2O5/ K2O) application rates were 98/155/98-123/195/123, 213/187/307-271/239/392, 400/398/631-473/470/ 746, and 17/101/251-21/123/306 kg/ha, respectively. This study provides a quantitative basis for designing more effective irrigation and fertilization strategies for sustainable citrus cultivation in Southwest China.