Goldspur Apple (Malus pumila cv. Goldspur) is one of the main fruit trees planted on the Loess Plateau in a semi-arid region. Gas exchange parameters in leaves of ten-year-old trees were studied under different soil water conditions with a LI-6200 portable photosynthesis system and a LI-1600 portable steady state porometer, in order to explore the effects of soil water stress on the photosynthesis and the suitable soil water content (SWC) for water-saving irrigation of the apple orchard. The experimental site is located in Tuqiaogou watersheds, Yukou town, Fangshan county, Shanxi province, China, a part of a gully-hilly area of Loess Plateau in the middle reaches of the Yellow River. It lies at 37° 36′ 58″ N, 110° 02′ 55″ E, with an average altitude of about 1 200 m and a maximum altitude of 1 446 m. The average annual precipitation is 416.0 mm, with the precipitation in June, July, August and September being more than 70% of the total amount. Annual potential evaporation is 1 857.7 mm and the greatest evaporation occurs in April to June. The soil belongs to medium soil and lossal soil and the average soil bulk density is about 1.2 g·cm-3, and mean field capacity (FC) is approximately 21.0%. In the study on responses of gas exchange parameters to soil water, eighteen apple trees were selected as experimental samples and divided into six groups (three trees per group and signed by Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ and Ⅵ). The soil water gradient was obtained in six groups by providing different water supply. During soil water treatment, SWC measured with LNW-50A neutron probe on June 12, June 16 and June 20, 1999 were as follows: group Ⅰ (22.4%, 18.7% and 16.7%), Ⅱ (20.1%, 17.4% and 16.2%), Ⅲ (18.2%, 16.7% and 14.8%), Ⅳ (15.9%, 13.6% and 11.9%),Ⅴ (11.7%, 10.5% and 9.6%) and Ⅵ (6.2%, 5.6% and 5.0%). A LI-6200 portable photosynthesis system was used to measure the photosynthesis parameters including photosynthetic rate (Pn), stomatal conductance (Gs) and intercellular CO2 concentration (Ci), while at the same time transpiration rate (Tr) was measured with a LI-1600 portable steady state porometer. The water use efficiency (WUE) and stomatal limiting value (Ls) were calculated according to formula WUE=Pn/Tr and Ls=1-Ci/Ca. These gas exchange parameters were measured on the same day that SWC was observed on June 12, June 16 and June 20, 1999, and the duration of observation was at 9∶00-11∶00 am each time.The results showed that when SWC was within a range about 60%-86% of field capacity (FC), Pn and Tr were maintained in a relative steady state and higher level, but below 60%-86% of FC, both Pn and Tr decreased obviously with decreasing soil moisture. The SWC range to support WUE at a relative steady state and higher level was about 50%-71% of FC. When SWC was less than 48% of FC, Gs and Ls declined with decreasing of soil moisture, while Ci increased rapidly. Based on the analysis of stomatal limitation of photosynthesis using two criteria (Ci and Ls) suggested by Farquhar and Sharkey, it implied that the predominant cause of restricting Pn had turn into non-stomatal limitation under severe water stress. According to the main intention of water-saving irrigation for enhancing water use efficiency, it is concluded that the suitable range of SWC was about 50%-71% of FC, as the most severe degree of soil water stress tolerated for photosynthesis was about 48% of FC.