It is essential to accurately assess the carbon sink of Populus euphratica Oliv. (P. euphratica) forests, given their long-term sequestration ability, to understand the carbon balance in arid zones, yet quantitative studies are still lacking globally. This study applied an optimised Carnegie-Ames-Stanford Approach (CASA) incorporating biomass and unmanned aerial vehicle (UAV) data to examine the spatiotemporal heterogeneity of the net ecosystem productivity (NEP) in China's P. euphratica forests during 2000-2023 and to analyse the impacts of climatic factors, Ecological Water Conveyance (EWC) and land-use changes. Results show that: (1) overall, NEP demonstrated a volatile upward trend notwithstanding a post-2017 downturn, with seasonal maxima in summer and minima in winter; (2) significant spatiotemporal heterogeneity in NEP was observed, with contrasting trends of increase in the Tarim, Aksu, and Heihe River basins but decrease in the lower Qarqan River; (3) human activities have now surpassed climatic factors as the primary driver of increased NEP in poplar forests, with EWC benefiting 90 % of the implementation areas; (4) the cumulative NEP of China's P. euphratica forests increased by 1200.38 Gg C over the last 20 years, largely attributable to land-use change from unused land to P. euphratica forests and cropland. This research provides a robust NEP estimation, clarifies its spatiotemporal patterns and driving mechanisms, and demonstrates that poplar forests serve as significant carbon sinks in arid regions. These findings support the development of evidence-based conservation strategies and China's 'dual carbon' goals.