Abstract: Abstract: Citrus is one of main fruits in China and it yield is unstable due to seasonal drought in recent years. It is important to understand the mechanisms behind the citrus tree growth under the drought stress. In this study, we investigated the effect of water deficit on photosynthetic characteristics, fruit yield and water use efficiency under drip irrigation. The orchard was located in Chendu, Sichuan. The tree was 7-year-old Shiranui citrus. The average tree height was about 2.8-3.0 m and the diameter of the tree was about 10 cm. A total of 4 water deficit treatments were designed in 2 growing stage of citrus (fruit expansion stage (Ⅲ) and fruit maturity stage (Ⅳ)). Meanwhile, the treatment without water deficit was considered as the control with 100% irrigation amount. The 4 water deficit treatments included 85% of irrigation amount (slight water deficit, LD), 70% of irrigation amount (moderate water deficit, MD1), 55% of irrigation amount (mild water deficit, MD2), and 40% of irrigation amount (severe water deficit, SD)). The response of photosynthetic characteristics of citrus leaf to water deficit were observed by measuring variation of transpiration rate, photosynthetic rate, stomatal conductance, intercellular CO2 concentration, stomatal limits, non-stomatal limits and instaneous leaf water use efficiency. The fruit yield and water use efficiency based on irrigation and evapotranspiration were also determined. The results showed that the photosynthetic rate varied in a dual-peak curve during the day. The photosynthetic rate of mild and severe deficit was significantly lower than the CK but that of slight deficit treatment was not significantly different from the CK. The transpiration rate was increased then decreased during the day for all the treatments and it was only significantly lower for the severe and relative mild deficit treatments compared to CK. The intercellular CO2 concentration was not significantly different for the LD and CK treatments. The carboxylation rate was also similar for the LD and CK. The instaneous leaf water use efficiency of IV-LD was significantly higher than the CK and increased by 36.61% (P<0.05). With decreasing the irrigation amount the stomatal limits increased by 1.24%-20.11% and the non-stomatal limits increased by 2.17%-33.99%. When the treatment changed from MD2 to SD and from MD1 to MD2, the influential factor of citrus photosynthesis changed from stomatal factors to non-stomatal factors. The evapotranspiration decreased with increasing degree of water deficit. The evapotranspiration of water deficit treatments at fruit expansion stage and at fruit maturity stage decreased by 10.7%-36.3% and 6.7%-12.2%, respectively. The yield of LD treatments was not significantly from that of CK. However, compared to CK, the water use efficiency based on evapotranspiration of III-LD and IV-LD treatments was increased by 13% and 9.5%, respectively; the water use efficiency based on irrigation amount of III-LD and IV-LD treatments was increased by 11% and 6.87%, respectively. Thus, the slight water deficit could not only keep fruit yield but also increase water use efficiency, which was a suitable water deficit model for citrus. The study provides valuable information for understanding citrus tree growth in drought stress.