Abstract:The photosynthetic characteristics and the relationship of net photosynthetic rate, water use efficiency with the environmental factors and stomatal conductance in four new poplar clones (3#、6#、8#、9#) were studied. The results showed that the peak value of the diurnal courses of the photosynthetic rate generally appeared around 10:00, and then reduced gradually. The diurnal courses of transpiration rate in different months were different. The peak value of the diurnal courses of stomatal conductance generally appeared at 7:00~9:00. The total mean net photosynthetic rate(μmol CO2·m-2·s-1), transpiration rate (mmol H2O·m-2·s-1) and water use efficiency (mmol CO2·mol-1 H2O) had following order in June, July and August respectively: Clone 9#（8.53）>6#（7.21）>3#（6.47）>8#（4.98）; Clone 9#（3.74）>3#（2.76）>6#（1.76）>8#（1.47）; Clone 8#（4.77）>6#（4.35）>3#（2.99）>9#（2.40）. Clone 9# had higher net photosynthetic rate and transpiration rate, with lower water use efficiency. On the contrary, clone 8# had lower net photosynthetic rate and transpiration rate, with higher water use efficiency. Net photosynthetic rate and water use efficiency had a close relationship with the air temperature and relative humidity in 6#, and had a close relationship with the photosynthetic active radiation and stomatal conductance in 3#、8# and 9#.