Melon cv. ‘Qingwang’ was adopted to study the effects of regulated deficit irrigation on the photosynthetic characteristics,fruit yield and quality of melon under plastic house conditions.The treatments included the sufficient water supply during the whole growing period (control),low and moderated water deficits at the stages of vine-extending or fruit-expanding.The results showed that compared with the control,low water deficit treatment at the stage of fruit-expanding had no significant effect on the net photosynthetic rate and fruit yield,increased the transpiration rate and stomatal conductance by 8% and 75%,respectively,decreased the amount of irrigation water by 6%,increased the water use efficiency and fruit firmness by 6% and 10%,and increased the ratio of total soluble solid/titrable acidity.Compared with the control,low and moderate water deficit treatments at the stage of vine-extending and moderate water deficit treatment at the stage of fruit-expanding decreased fruit yield by 25%,24%,27%,respectively,and decreased water use efficiency by 19%,11%,18%,respectively.Comprehensively considering the effects of water deficit treatments on each indicator measured in melon,low water deficit treatment during the fruit-expanding stage can improve the fruit quality without reducing yield,and increase water use efficiency.Thus,low water deficit treatment during the fruit-expanding stage has the potential to be popularized.