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The dominant climate-controlling system of the southwestern Gobi Desert and its surrounding areas in China at interannual to centennial time scale has not been determined. It is necessary to improve our understanding of the humidity variations in the past and forcing mechanisms in the southwestern Gobi Desert to enhance the accuracy of climate model predictions and cope with future desert advance/retreat crises resulting from climate change. We measured the annual stable oxygen isotope ratio (δ18O) in the cellulose of four Picea crassifolia trees growing on the fringe of the southwestern Gobi Desert from 1806 to 2011. Our tree-ring cellulose δ18O values (34.49‰) were considerably higher than those reported in other regions of Asia, suggesting that the southwestern Gobi Desert has been extremely arid over the past two centuries. The chronology of tree-ring cellulose δ18O is significantly (p < 0.01) negatively correlated with summer (June, July, and August) relative humidity (RH) records from 12 meteorological stations nearby our study region (Table 1). Based on the high correlation (r = − 0.740, p < 0.0001, n = 49) between tree-ring cellulose δ18O and instrumental summer RH from meteorological stations nearby our study region, we reconstructed summer RH variations during the past 206 years. Spatial correlation patterns between our reconstruction and several hydroclimate-related gridded data sets indicate that the reconstruction is representative of hydroclimate variations over a large area. Close agreement was observed between our reconstruction and several East Asian Summer Monsoon-related hydroclimate records. Our study indicates that the moisture from the east has introduced the RH to the southwestern Gobi Desert during the summer months over the past two centuries. [ABSTRACT FROM AUTHOR]