Abstract: In order to explore the response of photosynthetic physiological characteristics to different pruning intensities in Populus of middle and high forest age,10-year-old triploid Populus tomentosa asexual lines B301and S86 were used as materials,light pruning（pruning the lower 1/3 of the canopy） and heavy pruning（pruning the lower 1/2 of the canopy） and unpruned trees as the control were adopted respectively.The results showed that pruning had no significant effect on the growth rate of height,diameter at breast height and wood volume of 10-year-old Populus,but heavy pruning increased the net photosynthetic rate（Pn）,transpiration rate（Tr）,and stomatal conductance（Gs） of their remaining leaves and enhanced material synthesis at the single-leaf level,while pruning had no significant effect on instantaneous water use efficiency（WUE）.From a comprehensive analysis of growth and physiology,pruning the lower 1/2 of the canopy could improve the physiological condition of 10-year-old Populus tomentosa,but had no significant effect on the growth of the stand.