In this study, an experiment was carried out in accordance with the randomized complete blocks design. There are six replications in greenhouse between June 10th and August 20th in 2003. Four water treatments were designed. They were well-watered condition, slight water stress, moderate water stress and severe water stress. The irrigation intervals were strictly controlled. Under well-watered treatment, the seeding of the tested clones was irrigated every two days. For the others, the irrigation intervals were every three days, every four days and every five days respectively. However, the volumes of irrigated water were same every time, i.e. 500ml. The 10 tested clones were derived from a cross between the female Populus deltoides Bartr. CV. ‘Shanhaiguanensis’ and the male Populus deltoides Bartr. CI. ‘Harvard, which were named J1～J10. CK₁ and CK₂ were Populus euramericana CV. ‘114/69’and Populus deltoides Barter CV. ‘Zhonglin 46’, which perform well currently. The following physiological parameters were examined, the long term water use efficiency (WUEL), photosynthetic parameters (net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), instantaneous water use efficiency (WUE_i), maximum photosynthetic rate (P_max), light saturation point (LSP), carboxylic efficiency (Ce)), stomatal properties (stomatal frequency, stomatal size, superior/inferior ratio) and root/shoot ratio. The comparison of physiological parameters among different clones was studied under different water treatments. The interrelations between WUEL and photosynthetic parameters, stomatal properties, root/shoot ratio were analyzed. The results showed that the WUEL differences were obvious among tested clones, and the differences became greater along with the severer water stress. The WUEL differences of the tested clones under 4 water treatments were consistent. The best clone was J2, and J6, J7, J8, J9 were quite good. Some physiological parameters, such as stomatal frequency, stomatal size, Gs, Tr, Pn and root/shoot ratio of J₂ et al were moderate, while the superior/inferior ratio, Pmax, LSP, Ce of J2 et al were higher than other clones. Higher Pmax, LSP, Ce implied that J2 et al had strong photosynthetic capacity. Moderate Gs, Tr, Pn revealed that J2 et al not only had relatively higher WUE_i but also had relatively strong photosynthetic capacity. Moderate root/shoot ratio indicated that the photosynthetic products distribution was reasonable among root and shoot, which resulted to enough root to ensuring water supply and at the same time more photosynthetic products run up to canopy. The stomatal differences among tested clones might be a key factor which accounting for the photosynthetic capacity differences and WUEi differences, which ultimately induced to WUEL differences. The clones with moderate stomatal frequency and stomatal size had suitable Gs, Tr, Pn, and relatively higher WUEi. The clones with large superior/inferior ratio had strong photosynthetic capacity. The WUEL differences among tested clones were induced by a series of physiological parameters, and higher WUEL clones often had strong photosynthetic capacity, a comparatively higher WUEi, and a satisfied root/shoot ratio. For selecting higher WUE clones in breeding program, the co-operation of several physiological parameters should be concerned more than the effect of a single physiological parameter.