Tuyuhun and Tubo were two important states that thrived in north-western China during AD 311-900 in parallel with the Han Chinese dynasties of Sui and Tang periods. The Reshui Tomb Cluster located in Dulan County of the north-eastern Tibetan Plateau is an important cultural relic of the Tuyuhun-Tubo age. The official excavations of the Reshui tombs were regarded as top events in archaeology in the 1980s and 1990s in China. The Reshui-1 Tomb is the largest one among the tombs in the area. Since its excavation, there have been debates on whether the owner of the tomb belonged to the Tuyuhun or Tubo ethnicity. Therefore, accurately dating the Reshui-1 Tomb has a critical place in studying the Tubo and Tuyuhun histories. We collected 7 discs and 11 increment cores of Qilian juniper (Juniperus przewalskii Kom.) from the exposed and fallen beams of the roof of the Reshui-1Tomb. The lengths of the 16 tree-ring records are between 69 and 152 years. Based on a previously developed master dating chronology using Qilian juniper samples from the eastern Qaidam Basin, the calendar dates of the 16 specimens were determined by the COFECHA program and visual dating procedure. The average inter-series correlation among the dated sample series is 0.696, indicating good quality of cross-dating. The year of the outermost rings is AD 715 for the 7 discs and 4 out of the 9 increment cores. Moreover, the ring-width variations of the samples are consistent with the existing chronologies from the region. The presence of late-wood of AD 715 in the samples indicated that the Reshui-1 Tomb was completed in late AD 715 or early 716, which means that the Reshui-1 Tomb was finished in the Tubo age. This date provides direct evidence for archaeologists to determine the owner's ethnicity and identify of the Reshui-1 Tomb.