This paper takes the water shortage problem of vegetation construction of Platycladus orientalis, Pinus tabulaeformis, Robinia pseudoacacia, Populus simonii, Prunus armeniaca, Hippophae rhamnoides and Caragana microphylla these seven major tree species in Loess Plateau region as study object, using stem-flow instrument, WP4 soil-plant water potential instrument and Li-6400 photosynthesis measuring instrument system to study the dynamic change rule of soil moisture and water potential in different root-layers of different trees species, and water potential of leaves, vegetation transpiration rate, water using efficiency, photosynthesis characteristics and fluorescence characteristics in vegetation growing seasons of 2005 and 2006, in order to offer scientific basis for the tree choice and vegetation configuration of ecology construction in Loess Plateau area.1. Changing trends of monthly variation of soil moisture content are almost the same in different root-layers of different trees species, and are synchronization with the rainfall season distribution. Soil moisture content of June is the least as 2.6% — 3.6%, and that of August is the most as 8.0% — 9.5%. Soil moisture stock pile of Hippophae rhamnoides, Prunus armeniaca and Robinia pseudoacacia are relatively higher and more stable, while the changes of monthly variation of soil moisture stockpile of Caragana microphylla, Populus simonii, Platycladus orientalis and Pinus tabulaeformis are relatively larger.2. Changing trends of daily variation of water potentials of leaves and air are almost the same in different months of different trees species, all are presented a "V" form. While changing extends of water potential of soil, leaves and air are varied in time and space. Daily and monthly changing extends of water potential of soil and leaves are relatively smaller, while that of air are relatively larger. In space, water potential of air is much less than that of leaves and soil, the discrepancy between leaves water potential and soil water potential is not very large if compared with airwater potential.3. From July to September when the trees are growing the fastest, the average of water daily consumption of Populus simonii is 3.199 kg, the average consumption of water daily transpiration of single Caragana microphylla is 0.5677 kg, and the dissipation of water daily transpiration of single Hippophae rhamnoides is 0.152 kg.4. Under the natural rainfall condition, the changing trend of season variation of the tree transpiration is controlled by its own physiological regulation and growth rhythm, but not affected by soil moisture. According to water using efficiency, photosynthesis rate and transpiration rate, divide the tree species into four types, Robinia pseudoacacia, Populus simonii and Hippophae rhamnoides are water-saving and high-yield type, Pinus tabulaeformis and Platycladus orientalis are water-saving but low-yield type, Caragana microphylla is high-yield type but water- wasting type, and Prunus armeniaca is water-wasting and low-yielding type.5. Different trees species have different responses to light and CO_2 in different months. Accord to the different light responses, divide these seven research species into four categories. (1) Robinia pseudoacacia and Caragana microphylla are shade-tolerant species, which not only have strong ability to use weak light and are suitable for shade environment, but also can grow very well in full light environment. (2) Hippophae rhamnoides and Populus simonii are litmusless species, which can both be suitable for shade environment and grow well in full light environment (3) Pinus tabulaeformis and Prunus armeniaca are weak-light loving specie, which are growing better in weak light environment than in strong light environment. (4) Platycladus orientalis is light loving specie, which are growing very well in full light environment.6. According to Fv/Fm of different tree species in growing season, the order of drought-resistant ability for different trees is, from strong to weak, Caragana microphylla, Hippophae rhamnoides, Platycladus orientalis, Prunus armeniaca, Robinia pseudoacacia, Pinus tabulaeformis and Populus simonii.7. The PhiPS2, qP and ETR of Hippophae rhamnoides and Caragana microphylla are relatively higher, which means they have strong light- tolerant capability. The PhiPS2, qP and ETR of Platycladus orientalis and Pinus tabulaeformis are relatively lower, which means the light-tolerant capability of them are weak. Among the Robinia pseudoacacia, Prunus armeniaca and Populus simonii, the light-tolerant capability of Populus simonii is the strongest and that of Prunus armeniaca isthe weakest. According to the changes of the maximum value of plant NPQ, we can know that both the suitability for strong light and the self-protection ability to the threat from strong light of the leaves of Hippophae rhamnoides and Caragana microphylla are strong, and the order of suitability and self-protection ability of the other tree species is, from strong to weak, Platycladus orientalis, Populus simonii, Prunus armeniaca, Robinia pseudoacacia, Pinus tabulaeformis.