A severe drought affected the southern Tibetan Plateau (TP) in 2009 CE, causing substantial economic losses, yet its long-term context remains unclear. Here, we reconstructed early summer (Days 149-166) precipitation variability from 1720 to 2014 CE by using Juniperus tibetica tree-ring width indices (TRWI), whose radial growth is primarily controlled by early summer precipitation in the semi-arid southeastern TP. The reconstruction was validated against independent drought records and historical documentary sources. Although 2009 was the driest year since 1915 CE, more severe droughts occurred in 1735 and 1914 CE, highlighting the value of tree rings in extending drought history beyond the instrumental period. Using a 1.5-standard-deviation threshold, we identified 20 extreme wet years and 23 extreme dry years. Seven megadroughts were detected between 1737 and 1998, consistent with historical records. Megadrought frequency in the 20th century (three events) was higher than in the 18th and 19th centuries (two events per century), suggesting a possible increase under future warming. Based on daily correlation analysis, we also identified a daily precipitation threshold of 11 mm, above which tree growth sensitivity declines, indicating that TRWI-based reconstructions capture drought variability more faithfully than very wet extremes. These findings underscore the complexity of tree-growth responses and the importance of integrating multiple proxies-including documentary records and stable isotopes-for a more comprehensive understanding of hydroclimatic variability on the TP.