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We measured the annual variation in the stable isotopes of oxygen (delta O-18) and hydrogen (delta D) in tree rings of Abies georgei on the Batang-Litang Plateau of western China. Although correlations between tree-ring delta O-18 and delta D are relatively weak in semi-arid regions, we found a strong correlation between the delta O-18 and delta D time series from 1755 to 2009 under the wetter environment. Tree-ring delta O-18 and delta D time series are both significantly and negatively correlated with moisture conditions from June to August, including relative humidity and total precipitation, respectively, from 1960 to 2009. Considering the difference in low-frequency domain between the two isotopes, the relative humidity histories from June to August, reconstructed separately from the tree-ring delta O-18 and delta D data with instrumental climate data, reveal a persistent drying trend since 1850s, especially since the early 1970s. There is an obvious offset of reconstructed relative humidity from tree-ring delta O-18 and delta D in the period 1755-1820, despite the strong similarity in their 21-year moving averages. The decreased relative humidity since the 1850s may be associated with the thermal contrast between the sea surface temperature of the Indian Ocean and the Qinghai-Tibetan Plateau, which determines the strength of moisture transfer via the Indian summer monsoon.