Many previous studies have documented the relationship between the North Pacific Oscillation (NPO) and the weather/climate of upstream and downstream regions. However, the stability of NPO precursor signals of East Asian summer precipitation, which is important for climate prediction, has received little attention. This study identified temporal variations in the connection between the May NPO and subsequent midsummer precipitation over Northeast China (NEC) during 1961–2020. During 1986–2010, the correlation between the May NPO and midsummer precipitation over NEC was found significantly positive, whereas the relation was found statistically insignificant during 1961–1985/2011–2020. Further results indicated that the NPO stimulated a Rossby wave source over the North Pacific that was stronger during 1986–2010 than during 1961–1985/2011–2020. The Rossby wave source anomalies shifted to North America and to the North Atlantic during the subsequent June and midsummer, respectively. Consequently, a teleconnection wave train, which originated over the North Pacific, propagated eastward to North America, across the North Atlantic, and on to East Asia on the subseasonal time scale, influencing midsummer precipitation over NEC through modulation of the atmospheric circulation (e.g., horizontal wind, moisture transport, and vertical movement). Moreover, the intensified Rossby wave anomalies associated with the May NPO were attributed to the strengthened NPO, which could be related to the enhanced standard deviation of SLP over the North Pacific. Additionally, the strengthened out‐of‐phase relation between the NPO and the North Atlantic Oscillation, resulting from changes in the background circulation, partially contributed to intensification of the Rossby wave train. Plain Language Summary: The North Pacific Oscillation (NPO) is an important intrinsic atmospheric variability which is characterized by a north–south seesaw in sea level pressure over the North Pacific Ocean. The NPO has substantial influences on the weather/climate conditions over North America and East Asia. However, the relation between NPO and East Asian climate is not stationary. This study identified a temporal variation in the connection between the May NPO and the midsummer precipitation over Northeast China (NEC) during 1961–2020, which shows a strong (weak) correlation during 1986–2010 (1961–1985 and 2011–2020). This is attributed to the different atmospheric circulation patterns caused by the NPO during the three periods. The NPO can simulate stronger perturbation to the atmosphere during 1986–2010 than during 1961–1985 and 2011–2020. Consequently, the perturbed atmospheric waves can propagate eastward to North America, across the North Atlantic, and toward East Asia during 1986–2010, leading to anomalous atmospheric circulation in East Asia and influencing the midsummer precipitation over NEC. The strengthening of the NPO and the enhanced relation between the NPO and North Atlantic Oscillation during 1986–2010 can explain the above changes. This study may deepen our understanding of the influence of NPO on the East Asian climate and may be meaningful for climate prediction. Key Points: The effect of May North Pacific Oscillation (NPO) on summer rainfall at Northeast China (NEC) is not stable, with a strong (weak) relation for 1986–2010 (for 1961–1985 and 2011–2020)This is attributed to a stronger Rossby wave source stimulated by NPO during 1986–2010 than during 1961–1985 and 2011–2020The strengthened Rossby wave related with NPO is attributed to the intensification of NPO and to the enhanced relation between NPO and North Atlantic Oscillation [ABSTRACT FROM AUTHOR]