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.