Based on the typhoon data and NCEP/DOE AMIPII reanalysis data from 1979 to 2005, a study is performed of the behaviors of tropical cyclones generated in the monsoon trough (MTTC) in the South China Sea (SCS) and western North Pacific (WNP) as well as the effects of the monsoon trough intensity on MTTC. Analysis suggests: (1) The total number of tropical cyclones (TCs) is 672 over the SouthChina Sea and the western North Pacific during May to October from 1979 to 2005, of which MTTC is 491, making up 73.1% of the total and 79.2% of the onland typhoons. The grasp of the regularity of MTTCs activities means the grasp of the principal regularity of the western North Pacific TCs, which are TCs mainly affecting China. (2) During this period, the annual occurrence frequency variation of MTTCs experienced three stages: normal (1979 to 1987), more MTTCs (1988 to 1994), and fewer MTTCs (1995 to 2005); there was periodic variation in MTTC with quasifouryear and quasitwoyear periods, with 1994 being the turning point. (3) In the years of anomalous MTTCs, there was great difference in onset/ending dates and generation positions. In the years of fewer/more MTTCs, in comparison with the mean condition, MTCCs started later (earlier) and continued to be in inactive (active) phase from May to October with a smaller (larger) generation area. (4) The monsoon trough intensity and position affect greatly the generation position and annual number of MTTCs.