Estimation of the ecological water demand of natural plants is a key issue for the restoration and reconstruction of a fragile ecological region in arid areas. Tree rings of Populus euphratica in the Yingsu section of the lower reaches of the Tarim River and their responses to climate change and hydrological conditions were analyzed using the nonparametric Mann-Kendall trend test, Mann-Kendall-Sneyers test, moving t-test, one-way ANOVA, and Pearson correlation. The ecological water demand of a desert riparian forest in the Yingsu section was then estimated based on the tree ring characteristics using regression and stepwise regression models. The results showed that the abrupt temporal changes of temperature and precipitation were different from that of the standard chronology of P. euphratica in the lower reaches of the Tarim River. Moreover, there was no significant relationship between temperature and standard chronology of P. euphratica or between precipitation and standard chronology of P. euphratica in the lower reaches of the Tarim River. The Pearson correlation analysis also revealed no significant relationship between temperature and groundwater depth or between precipitation and groundwater depth in the lower reaches of the Tarim River. However, the width indexes of tree rings of P. euphratica were significantly and negatively related to the groundwater depth, and they were significantly and positively related to the amount of ecologically conveyed water from August to July in the Yingsu section of the lower reaches of the Tarim River. These results proved that historical information regarding water conditions could be accurately reproduced by the tree rings of P. euphratica in the lower reaches of the Tarim River. Therefore, the tree rings of P. euphratica could be used for quantitatively assessing the restoration effects of ecological water conveyance to desert riparian forests and calculating the ecological water demand for natural vegetation in the lower reaches of the Tarim River. The average contribution of ecological water conveyance to the increased tree ring width index was 42.96% in the Yingsu section of the lower reaches of the Tarim River. Estimated from the equation of the tree rings of P. euphratica and the amount of ecologically conveyed water, an annual ecological water conveyance of 0.84×10⁸ m³ was expected to maintain the average growth (i.e., average tree ring index of 0.997 during 1933-2015) of P. euphratica in the Yingsu section within a distance of 300 m to the river channel in the lower reaches of the Tarim River. Furthermore, an annual water conveyance of 0.91×10⁸ m³ was estimated to keep P. euphratica at an average radial growth rate of 0.972 during the period of 1933-1974 (i.e., before the main channel of the Tarim river near Yingsu section dried up).