Abstract: Based on ERA5 (the fifth major global reanalysis produced by European Centre for Medium-Range Weather Forecasts) and NOAA (National Oceanic and Atmospheric Administration) reanalysis data from 1979 to 2020, this study investigates the interannual variability of the summertime QBW (quasi-biweekly) East Asia–Pacific (EAP) teleconnection and the modulation effect of mid-latitude Eurasian wave trains. The results of the study reveal that the interannual variation in the activity of QBW EAP events primarily stems from differences in intensity rather than frequency. QBW EAP events in strong and weak years exhibit different magnitudes of the WNP (western North Pacific) dipole anomaly and display distinct upstream patterns over the Eurasian mid-latitudes. The leading modes of the upper-level meridional winds over the Eurasian mid-latitudes were extracted using EOF (empirical orthogonal function) analysis. The first mode, characterized by a single-wave train structure, is significantly correlated (leading by 3 d) with QBW EAP development in weak years. The second mode, exhibiting a dual-wave train structure, shows a significant lead correlation (leading by 4 d) with QBW EAP development in strong years. During the development and mature phases of QBW EAP events in strong years, the upstream dual-wave train in the upper troposphere disperses energy to the WNP, which in turn promotes enhanced coupling with the lower-level circulation, fueling QBW EAP events. Conversely, in weak years, the upper-level single-wave train demonstrates minimal influence on the lower levels and decays before the maturation of QBW EAP events, thus playing a minor role. Moreover, compared with the single-wave train, the dual-wave train is more favorable for triggering potential vorticity intrusion and wave-breaking events near the jet exit region, which in turn enhances WNP convection and promotes QBW EAP development. Thus, it can be inferred that the dual-wave train in strong years results in stronger QBW EAP events through phase superposition and wave-breaking processes compared with the wave train in weak years.