Abstract: In this paper, the relationship among the southern branch of the Easterly Jet(SEJ) at the 150 hPa lever near 100°E.Swmonsoon and the activities of typhoon over the South China Sea and western North Pacific is discussed.By using monthly data in August of 1968-1981, it is found that when SEJ is strong(weak), the number of typhoon occuring in August will be more (fewer) than monthly mean value.The calculated results based on objective anlysis show that the dynamic structures are quite different from each other in the months when SEJ is strong or weak. During strong months, the monsoon longitudinal cell locates more poleward and is stronger, the upward motion in the area north to SEJ over the South China Sea and western North Pacific is nore intensive than those during weak months. So it favours the occurrence of typhoon.Furthermore, the daily data of summer in years of 1979-1981 are analysed in detail. The result shows that when SEJ is strong (weak), the SW monsoon on the 850 hPa level south of 10°N over the South China Sea is correspondingly strong (weak) During the period they both are strong,most of typhoons occur. In addition, when SW monsoon over western Pacific is strong, typhoon may occur extremely frequently Finally, It is found that the V component of SEJ directing south provides the way for the upper mass exporting over the tropical disturbance, which will favours the occurrence and development of typhoon.