Photosynthesis and water use are the most important factors limiting photosynthetic assimilation of carbon dioxide and growth of individual plant in terrestrial ecosystems. To understand the role of photosynthesis and water use in plant of the desert-oasis ecotone, this study took Tamarix ramosissima-a native dominant desert shrub-as a subject to investigate the plant adaptability to light and precipitation under different controlled conditions. The experiment was conducted within the desert-oasis ecotone in the southern periphery of Gurbantunggut Desert with four treatments. The treatments are A: natural state without any disturbance, B: simulated precipitation with 12 mm which is equivalent to the average of the maximum precipitation event within September last five years taking place in target desert-oasis ecotone, C: shading, i.e. the whole plant covered by opaque black cloth and D: a combination of treatments B and C. Using the LI-6400 system by LI-COR, Inc and Model 3005 plant water pressure chamber, the authors studied the responses of photosynthesis and the variation of water potential of branches to the simulated precipitation and light variation, and investigated the diurnal course of water use efficiency (WUE) and light use efficiency (LUE) of T. ramosissima. Results showed that the treatment B resulted in rich water content ranging from 15% to 43% in shallow soil layer within the depth of 0-40cm, depending on the evaporation, but water content in the soil layer between 40cm and 100cm does not exit significant different from in natural state, which indicates that water from precipitation can only infiltrate to the depth of 40cm of the soil under the condition with the maximum precipitation event. There is slight difference in variations of the water potential in branches with changes of surface soil moisture under the four treatments. The highest and lowest water potential in branch presented in predawn (-1.2 MPa) and midday (-3.2 MPa) respectively, and then the water potential in branch gradually increased after the sunset. The diurnal variations of WUE and LUE in T. ramosissima showed bimodal pattern. The diurnal courses of WUE and LUE under shading conditions was lower 1.5 μmol CO₂/mmol H₂O and 0.20×10^-2μmol CO₂/μmol APAR than those under natural conditions with the variation of light, respectively. It is demonstrated that even the shallow soil is under an extreme drought condition, water potential and photosynthesis of T. ramosissima branches are still able to stay at a normal level. A lower water potential indicates that T. ramosissima is able to survive under extreme drought stress. Physiological activity and biomass accumulation of T. ramosissima rely on the stable groundwater, and variations of soil moisture in the upper soil layers has slight influence on its photosynthesis and evapotranspiration. The "photosynthetic descent" phenomenon of T. ramosissima is caused by the high light and temperature in midday, which reflected the ecological adaptability of T. ramosissima in the course of cooperative evolution with the environment. Furthermore, the sensitive regulation of stomata was the adaptability mechanism in response of T. ramosissima to the variation of light and droughty habitats. These phenomena are significantly related to the water use strategy, leaf morphology, root function and eco-physiological adaptation of T. ramosissima. However, the groundwater table decreased in some transition regions between deserts and oasis over the past 50 years. As a result, T. ramosissima is difficult to obtain underground water and can not effectively adjust the individual ecological structure and water use strategies within certain period to adapt to changes in water conditions. They generate negative effect on phreatophyte species T. ramosissima, and even threatens its survival in the future. Therefore, it is suggested to prevent overexploitation of groundwater and over-harvesting T. ramosissima.