The researchers established the first annual resolution tree ring δ¹⁸O chronology of Qilian juniper (Sabina przewalskii Kom.) for the eastern margin of the Qaidam Basin, northwestern China. The mean tree ring cellulose δ¹⁸O in the study region (33.46‰ ± 0.99‰) was higher than that in the Qilian (28.7‰) and Helan Mountains (27.66‰), northwest China, and higher than the values in other published research (15.0‰ to 33.0‰), possibly as a result of the heavier δ¹⁸O in precipitation and the lower relative humidity (43%) during the growing season. The results of our simple correlation, bootstrap correlation, and response analyses showed that the cellulose δ¹⁸O was negatively and significantly correlated with relative humidity in July and August and that this correlation explained about 31.4% of the total variance in the moisture variability from 1954 to 2006. We also detected significant correlations among the Palmer drought severity index, the standard precipitation index, and the tree ring δ¹⁸O series. These analyses revealed that the summer relative humidity was the main limiting factor that affected δ¹⁸O values in the tree ring cellulose. Previous research indicates that the direct influence of the East Asian summer monsoon (EASM) reaches as far west as 100°E in China. However, our study revealed that the tree ring δ¹⁸O series was significantly correlated with the EASM index from 1873 to 1975, which means that the EASM influences moisture variability in the Qaidam Basin (farther west than 100°E) and that the tree ring δ¹⁸O chronology we obtained can be used as a proxy to study historical variability in the EASM.