To uncover the effect of mulched drip irrigation under water deficit on the yield and quality of onion in Hexi corridor,a field experiment was conducted taking "Hongbao 903" as study object. The experimental employed a two-factor randomized blocks design of mulched drip irrigation with different water deficit gradients in growth periods,which included nine treatments totally. Onion photosynthetic parameters in every growth period were measured,and the onion productivity and quality were determined after harvest. Results showed that: (1) Water deficiency (WD) in the development and bulbification stages obviously decreased net photosynthetic rate,transpiration rate and intercellular CO₂concentration,and the reduction increased with the raise of water deficiency. (2) WD decreased yield in different degree. Slight water deficit in ripening stage was the highest (47 940 kg/hm²),secondly in the establishment satge (46 521 kg/hm²),of which decreased by 5.93% and 8.71% (P>0.05) respectively compared with full irrigation. (3) Slight water deficit in the establishment period gave the highest irrigation efficiency (58.84 kg/m³),secondly in ripening stage (52.60 kg/m³),of which increased by 36.14% and 21.70% respectively compared with full irrigation. (4) The onion quality was the best under the treatments of slight water deficit in the establishment stage,indicated by the increased compactness,onion oil,soluble sugar,soluble protein,vitamin C and pyruvic acid,by 3.06%,2.50%,10.40%,17.57%,4.35% and 6.06% respectively,compared with full irrigation. Taking the productivity,water product efficiency,output value and fruits quality,etc. altogether,the principal component analysis was used for comprehensive evaluation. The slight water deficit in the establishment stage was an ideal stage for soil moisture regulation. There lative soil water content should be kept around 70%~75% in the establishment stage,and 80%~85% in others. These results could provide the theoretical and technical references for the water saving planting ofonion and its industrialization development.