In recent times, the warming and humidification of the Tibetan Plateau have become more pronounced. However, research on soil moisture on the Tibetan Plateau has been limited by the late initiation of data acquisition, thereby constraining the study of its driving mechanisms. Hence, there is an urgent need for historical time series to fill this research gap. To address this issue, we analyzed four spruce (Picea likiangensis) chronologies collected in high-altitude regions of the Tibetan Plateau, and we found that soil moisture during the dry season was a critical limiting factor for tree growth. The first principal component of a principal component analysis, accounting for 56.21% of the variance in all chronologies, was then used to reconstruct soil moisture variations from October of the previous year to May of the current year in the southeastern Tibetan Plateau from 1778 to 2020. We found significant relationships between soil moisture changes and the El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Variability (AMV). We also found a strong linkage between dry season soil moisture and summer temperature, suggesting that this relationship contributes to the intensification of warming and humidification on the Tibetan Plateau.