Abstract: Abstract: In order to effectively improve the water-use efficiency of apple trees and rationally allocate irrigation water, this study selected the mountainous apple trees located in the loess plateau of northern Shanxi province (37° 27’ N, 110° 2’ E, altitude: 1 020 m) as the research object, and studied the effects of water deficit on the growth, fruit quality, yield and water use efficiency of apple trees under surge-root irrigation (SRI) through 3-year field experiments from 2016 to 2018. Light (L), moderate (M) and severe (S) water deficits were carried out at the germination stage (I), 2 levels of L and M were designed at fruit-bearing stage (II) and fruit-filling stage (III) respectively, full irrigation (FI) and non-irrigation (NI) were designed as the control during the whole growth period, with a total of 9 treatments. The results showed that the evapotranspiration of various stages on apple trees were in the following descending order: stage III, stage II, stage I and fruit maturity stage (IV). Evapotranspiration in stage III accounted for 59% to 71% of the total in the whole growth period, which was much higher than that in the other 3 growth stages. Compared with FI, the water-saving effect was significant (P<0.05), and the growth of new apple shoots was inhibited to some extent. The treatment with a large degree of moisture deficit reduction had larger effect of over-compensation after sufficient irrigation in the next stage. Compared with FI, water deficit treatment during the different growth stages had significant effects on the fruit quality, fruit yield, and water use efficiency (WUE) of the apple trees. The effects of water deficit on fruit water content, soluble solid content and soluble reducing sugar content in the I stage were not significant (P>0.05). Water deficit in the III stage increased fruit firmness, reduced single fruit weight, excellent fruit percentage and fruit water content, so the III stage was not suitable stage to start water deficit treatment. Water deficit in the II stage of apple trees slightly increased fruit firmness, fruit water content decreased slightly, and single fruit weight, excellent fruit percentage, soluble solid content, and soluble reducing sugar content of apples were significantly increased, which made apples sweeter and easier to store. Water deficit in the I stage of apple trees had no significant effect on apple yield (P>0.05). However, water deficit was significantly reduced yield in the III stage of apple trees (P<0.05). In 2016, the yield of apple trees by III-L and III-M treatments decreased by 10.89% and 13.46% respectively compared with FI and decreased by 3.66% and 10.10% in 2017, respectively, decreased by 10.58% and 13.42% in 2018. The yield of apples with water deficit during the II stage was significantly higher than that of FI (P<0.05). The WUE of the apple trees was higher with II-L and II-M treatments (greater than 7 kg/m3) than with the other treatments. The fruit quality, yield and water-use efficiency of apples with the II-M treatment were higher among all the treatments. Therefore, the optimal treatment of water deficit should be II-M. The study could provide technical support for apple water management and precision irrigation in mountainous areas of northern Shaanxi.