Abstract: Precipitation on the Qinghai–Xizang Plateau (TP) has garnered attention owing to its critical role in the regional water cycle. An analysis of data from 96 meteorological stations from 1961 to 2017 on the eastern TP examines the relationship between the leading mode of summer (JJA) precipitation variability and the North Atlantic Oscillation (NAO), along with the mechanisms behind their unstable relationship. The results show that the NAO significantly influences the leading mode of summer precipitation on the eastern TP, with interdecadal variations in their relationship. During the early period (1961–1993), the NAO induced a north-south dipole pattern, while in the later period (2002–2017), it led to a monopole pattern. The NAO can generate potential height anomalies with a quasi-barotropic structure in the upper layers over the TP, forming corresponding circulation anomalies in the lower layers, which in turn affect regional water vapor anomalies. During the early (later) period, the western edge of the NAO-related circulation anomalies was located in the northeastern (northwest) TP, facilitating the formation of precipitation anomalies in a north–south dipole pattern (monopole pattern) in the eastern TP. The shift in the leading modes of summer precipitation resulted in an abrupt change in the NAO-precipitation relationship in 1998. Given the nonstationary relationship between NAO and summer precipitation on the eastern TP, it is essential to consider the interdecadal background signal when studying the NAO’s impact on regional climate. This is crucial for improving prediction accuracy and model simulation capabilities.