Water is a decisive factor limiting plant growth and development in extremely arid areas. Although numerous studies on the relationship between water resources and plants in the lower reaches of Tarim River, Xinjiang, China have been published, most of them focused on short time changes in the physiological or biochemical characteristics of individual plants, plant diversity, or normalized differential vegetation index. Thus, the relationship between long-term plant growth characteristics and water resources must be examined further. In this study, the responses of the long-term growth characteristics of Populus euphratica to ecological water conveyance from 2000 to 2015 in the lower reaches of Tarim River were analyzed using the tree-ring width chronology and stable carbon isotope (δ ¹³C) characteristics of the species. Results showed that: (1) P. euphratica growth was significantly related to the amount of ecological water conveyance, and the tree-ring width chronology and δ ¹³C values of P. euphratica in Yingsu indicated three stages. Specifically, tree growth was very sensitive to ecological water conveyance in 2000—2003, and the main tree-ring width chronology and δ ¹³C values of P. euphratica in this period were 1.52 and -26.70‰, respectively. When ecological water conveyance was reduced in 2004—2009, the mean tree-ring width chronology and δ ¹³C values of the species decreased by 28.83% and 2.41%, respectively. The values of these indices remained stable when the ecological water conveyance amount was increased and maintained in 2010—2015. (2) P. euphratica growth was closely related to the ecological water conveyance time, especially in terms of growth year (from previous September to current August), growth season (from April to August), and previous growth year (from previous April to current March). However, plant growth was not significantly related to ecological water conveyance of the calendar year (from January to December). Because no ecological water conveyance was conducted in the growth season, growth year, and previous growth year of 2009 or in the growth seasons of 2006—2009, the tree-ring width chronology and δ¹³C values of P. euphratica of 2009 were lowest in 2000—2015. The tree-ring-width chronology and δ ¹³C values of P. euphratica in 2009 decreased by 61.18% and 3.14% compared with those in 2003, respectively. (3) Ecological water conveyance decreased the groundwater depth and improved the water environment of P. euphratica, thereby enhancing the water use efficiency, accumulated photosynthetic production, and growth of the trees. However, a 1-year lag effect between ecological water conveyance, groundwater depth, and P. euphratica growth was also observed. Therefore, ecological water conveyance should be implemented during the plant growth period in the lower reaches of Tarim River to restore and reconstruct the desert riparian forest effectively.