Abstract: The effects of soil drought degree and duration and gradually drought course on gas exchanging parameters of poplar leaves and response to light were studied by the test of potted plant with 3-years-old Poplar （Populus alba×Populus berolinensis）. The results indicated that poplar presented strong drought resistance and moderate drought was favorable to photosynthesis. In given soil moisture content （40.1%^-20.2%）, the rate of photosynthesis decreased with the soil moisture and the decreased degree was smaller and apparent quantum efficiency kept stable. The rate of photosynthesis decreased tempestuously when the soil moisture lowered to 15.5%. The light saturation point of Poplar is between 750 to 1300μmol.m ^-2 .s ^-1 and reduce with the reducing of soil moisture content. There was clearly a close positive correlation between photosynthesis rate, transpiration rate and stomatal conductance, and negative correlation with intercellular CO₂. Water use efficiency （WUE） increased with the water moisture content in soil when gradually drought course, but seriously drought stress in long term resulted in decreasing of WUE. Under the same condition the effect of water content gradual drought stress to photosynthetic rate was weaker than that of gradient drought, which showed that the poplar could endure temporal drought. Serious water stress lasting long time could lower rate of photosynthetic and transpiration greatly, which reflected the adaptive characteristic of poplar.