Abstract: The Three-North Shelter Forest is an important ecological defense of Beijing-Tianjin-Hebei area. About four-fifths of poplar plantations declined and about a third of them were dying or already dead in the last ten years. The mortality of trees resulted in the decline of ecological function of poplar plantation. In this study, we investigated the differences of δ¹³C and intrinsic water-use efficiency (WUE_i) values in the tree-ring between dieback and non-dieback trees with stable carbon isotope method. The cause of poplar plantation degradation and mortality was retrospectively ana-lyzed. The results showed that the diameter of poplar trees of nearly the same age decreased with the increase of degradation degree. The δ¹³C value of rings from dieback trees varied between -25.26‰ and -22.97‰, whereas that of non-dieback ones was -26.15‰ to -23.50‰. The δ¹³C values of dieback trees were higher than that of non-dieback ones from 1997. There was a nonsignificant difference of the WUE_i between dieback and non-dieback tree-ring from 1997 to 2001. And the difference of WUE_i between dieback and non-dieback tree was significant since 2002. The continuous occurrence of positive ΔWUE_i(WUE_idieback-WUE_inon-dieback) values might be one of important factors for subsequent divergence of the dieback and non-dieback poplar trees. The WUE_i of both dieback and non-dieback trees had no significant relationship to precipitation, relative humidity and ET₀, while significantly related with air temperature and ground water depth. The retrospective ana-lysis results showed that extreme drought in 1997 was a threshold when poplar trees began to decline. The underground water was overused because of land use change, which increased the intensity and duration of the drought, thus accelerated the degradation and mortality of poplar trees.