The progressive warming and humidification of eastern Central Asia (northwest China) over recent decades has attracted significant attention; however, our knowledge of climate evolution in this region prior to the instrumental era and its driving mechanism is limited. In this paper, we establish a tree-ring stable oxygen isotope (delta O-18) chronology of Picea schrenkiana growing on the southern slope of the Central Tianshan Mountains since 1618 CE. Tree-ring delta O-18 exhibits significant negative correlation with summer relative humidity (May-June; r = -0.69, p < 0.001, n = 58), and based on the reliability of this relationship, we reconstruct a 358-year summer relative humidity history. Our reconstruction shows that regional hydroclimate has been getting slightly wetter at the inter-decadal time scale over the past four centuries, implying that the humidification observed in meteorological records since the 1950s in eastern Central Asia may be part of a longer-term trend. Further analysis indicates that the hydroclimatic variability in the southern Central Tianshan Mountains is primarily driven by coupled North Atlantic ocean-atmosphere processes variations on decadal scales. This study provides centennial-scale empirical evidence supporting a humidification trend in eastern Central Asia, revealing the key role of the Atlantic teleconnections in the regulation of water vapor in inland arid areas.