Abstract: The influence of Northwest Pacific summer monsoon (NWPSM) characteristics and th e structure of monsoon trough on typhoon genesis is studied. When the NW Pacific monsoon trough strengthens and stretches eastwards, causing the enhancement of NWPSM, the vertical wind shear, divergent winds in the upper and lower atmosphere, humidity and sea surface temperature are all favorable to the typhoon genesis in the NW Pacific, and the number of typhoon genesis is evidently more than tha t when the NW Pacific monsoon trough is weak. Furthermore, the strength of the N W Pacific monsoon trough also has great impact on the locations of typhoon genes is. Typhoon geneses mostly occur in the western Northwest Pacific in the strong phase of the monsoon trough, whereas in the eastern Northwest Pacific in the wea k phase. These show that the NWPSM system impacts typhoon genesis mainly through the activities of the monsoon trough. And the strength of the summer monsoon al so has influence on typhoons. When the NWPSM strengthens in the active phase, th e number of typhoon geneses is more than that in the break phase. On the other hand, the NWPSM has impact on typhoon activities through intraseaso nal oscillations, and the 30-60 day oscillation is the dominant component of int raseasonal oscillations. The number of typhoon genesis is obviously more in the wetphase of low frequency convection, and less in the dryphase. The west win d phase of the low frequency zonal wind is favorable to typhoon genesis, and the t yphoon genesis is less in the east wind phase. The cases on 4 and 7 Septembe r 1987 also clearly reveal that the typhoon genesis is influenced by intraseason al oscillations, especially by the 30-60 day oscillations. In addition, the feat ures of NW Pacific summer circulation in the excessive typhoon (group occurrence ) period are analyzed. In these periods, there are generally strong monsoon trou gh, weak vertical wind shear and sufficient water vapor supply, and vice versa i n deficient typhoon periods, The above indicates that the temporalspatial grou p occurrence of typhoon genesis in the NW Pacific has a close correlation with th e activities of the NWPSM.