Hydrological processes of the Poyang Lake are significantly altered under the influences of human activities and climate changes. Ecological system has aological alterations indapted to the hydrological conditions before hydrological alterations in the Poyang Lake. Given this

we use various kinds of alterations diagnosis methods to test the lake water level. Then we use variable bandwidth kernel density estimation to fit respectively daily water level for each month before hydrological variations. We regard largest probability of water level as the ecological-level the Poyang Lake. The following results were obtained: ① The variation of water level in the Poyang Lake  occured  in 1979

  with the increase of 0.5 meter in the mean water level. ② Variable bandwidth kernel density estimation is the best statistical method for studying of maximize density water level before hydrological alterations in the  Poyang Lake. ③ After  the hydrological variation

the fill rate of ecological-level reduced significantly. with an  average value of ecological-level fill rate less than 50%. Runoff changes are the major factors influencing the ecological-level fill rate; Water conservancy projects and forest coverage can help to improve the low-flow

so the ecological-level fill would increased significantly in dry seasons after alterations. This research results will provide important scientific basis for the ecological conservation

water ecology and water management within in the Poyang Lake basin in a changing environment.