Abstract: Based on ERA5 and NOAA reanalysis data from 1979 to 2020, this study investigates the interannual variability of the summertime quasi-biweekly (QBW) East Asia-Pacific (EAP) teleconnection and the modulation effect of mid-latitude Eurasian wave trains. We found that the year-to-year variation in the activity of QBW EAP events primarily stems from differences in intensity rather than frequency. The QBW EAP events in strong and weak years exhibit different magnitudes of the western North Pacific (WNP) dipole anomaly and display distinct upstream patterns over the Eurasian mid-latitudes. We extracted the leading modes of the upper-level meridional winds over the Eurasian mid-latitudes using EOF analysis. The first mode shows a single wave train structure, which is significantly correlated with circulation anomalies during the development phase of QBW EAP events in weak years. In contrast, the second mode exhibits a dual wave train structure, significantly correlated with circulation anomalies during the development phase of QBW EAP events in strong years. In strong years, during the development and mature phases of QBW EAP events, the upstream dual wave train in upper troposphere disperses energy to the WNP and couples closely with the lower-level circulation, fueling the QBW EAP events. Conversely, in weak years, the upper-level single wave train has barely influence on the lower levels and decays before the maturation of QBW EAP events, thus playing a minor role. Moreover, the dual wave train in strong years promotes more WNP convection via wave-breaking processes than the single wave train in weak years. Thus, the dual wave train in strong years leads to stronger QBW EAP events through phase superposition and wave-breaking processes.