Abstract: Using best-track data of typhoon, the bimodal ISO (IntraSeasonal Oscillation) index and global daily outgoing longwave radiation data, we study the possible mechanism of the interdecadal variation of Boreal Summer IntraSeasonal Oscillation (BSISO) that has caused sudden decrease of multiple tropical cyclones (MTC) in the western North Pacific (WNP) after 1996/1997. The analysis shows that only the "MTC3" events, which contain 3 or more typhoons, abruptly decreased after 1996/1997. They are more likely to occur in the BSISO active phase with a slower propagation speed and longer low-frequency convection. Moreover, the MTC3 events have relatively low requirements for the BSISO amplitude. After 1996/1997, the eastward propagation range of the BSISO convection reduced, the period prolonged, and the days of BSISO active phase decreased, resulting in a significant weakening of low-frequency signals to the east of 145°E. Tropical WNP tended to remain in inactive phase for a long time during this period. When a typhoon pre-exists in the WNP, the southeastward-propagating Rossby dispersion wave train casused by it always dissipates without coupled low-frequency convection to the east of 145°E, which leads to the sudden decrease of MTC3 events after 1996/1997.