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Reconstruction of relative humidity changes in the upper Tarim River using carbon isotopic tree-ring chronology bridges the gap in historical observations on the Tarim River Basin in Arid Central Asia. <i>Populus euphratica</i> Olivier (<i>P. euphratica</i>), growing in the Tarim River Basin of Xinjiang, is an excellent record of past climate change. Based on precise dating, we analysed alpha-cellulose stable carbon isotopes in four cores of <i>P. euphratica</i> taken from the Alaer region of the upper Tarim River Basin. The four stable carbon isotope series records were corrected by the “pin method” and then combined into a carbon isotopic discrimination (Δ¹³C) series by the “numerical mix method”. The discrimination (Δ¹³C) series were clearly correlated with the mean relative humidity (RH_AS) in April–September of the growing season (<i>n</i> = 60, <i>r</i> = −0.78, <i>p</i> < 0.001), and according to the climate response analysis, we designed a simple regression equation to reconstruct the mean relative humidity (RH_AS) in April–September from 1824 to 2018 on the Alaer region. The reconstructed sequence showed mainly dry periods in the last 195 years, 1857–1866 and 1899–1907, while primarily wet periods from 1985 to 2016. Due to increased global warming and human activities, the climate shifted from “warm–dry” to “warm–wet” in the mid-to-late 1980s, when there were signs of a shift from “warm–wet” to “warm–dry” in the 2010s, with an increasing trend towards aridity. The RH_AS series of Alaer compares well to other hydroclimate series’ surrounding the research area, and the spatial correlation analysis indicates that the reconstructed series has good regional representativeness. On an interdecadal scale, the revamped RH_AS series is positively correlated with the Atlantic Multidecadal Oscillation (AMO) and negatively correlated with the North Atlantic Oscillation (NAO), reflecting the influence of westerly circulation on regional wet and dry variability. At the same time, the RH_AS may also be influenced by The Pacific Decadal Oscillation (PDO).