The seasonal and interannual variability of boreal summer and autumn monsoon trough in the western North Pacific (WNP) and their influence on the large-scale environmental factors of tropical cyclone (TC) formation are examined using the ERA-Interim reanalysis data during 1979-2012. The results suggest that a pronounced seasonal variability of monsoon trough over the WNP is shown. During June-July, the monsoon trough and deep convection over the South China Sea (SCS) and the western WNP in the 5°N-15°N latitudinal band, and are extended eastward. In August and September, the monsoon trough and deep convection shift toward the north and extend eastward and are located over the SCS and the western and central Pacific in the 10°N-20°N latitudinal band. In October and November, the weakened monsoon trough retreats westward and southward. There is an interannual variability in the monsoon trough over the WNP, During the strong (weak) monsoon trough years, the monsoon trough extends eastward (retreats westward). Seasonal monsoon trough activity is influenced by interannual monsoon trough variability. The change of large-scale environmental factors coincides with seasonal and interannual variability of the monsoon trough.