The regularity of Water use efficiency（WUE） of the variation in response to changes of environmental factors in popular（including P.deltoides cv.‘2KEN8’,P.deltoides cv.‘55/65’ and P.× euramiercana cv.‘Nl-80121’） ecosystem were studied by the eddy-covariance that has monitored the carbon and water flux between atmosphere and poplar ecosystem in bottomland of Yangtze River. The WUE of the ecosystem had regular diurnal variation decided by ecophysiological characteristics in different seasons. And environmental factors also affected the WUE obviously. (1) The WUE had a razor-thin decrease with the temperature increasing from April to September and increased from October to March in next year as the temperature increasing. (2) In the flood season, WUE increased in case of soil ponded, and changed irregularly under the non-ponded situation as the rising of SWC. (3) In non-flood season, the average of soil heat flux showed an positive correction with the average five-days WUE. (4) WUE and VPD significantly correlated negatively. By the principle analyze, SWC and air temperature had larger impact on the WUE in popular ecosystem.