By means of the conventional observation data and NCEP reanalysis data, the sustained torrential rain event caused by the severe tropical storm Bilis (0604) is investigated. The results show that the strong vapor transport and updrafts originated from strong convergence at the low level under the persistent interaction of the tropical storm and southwest monsoon the south of Bilis, as well as the zones of convergence at the low level, divergence at the high level, updraft and highintensity vapor convergence, are rarely reported for recent years, although Bilis is only a severe tropical storm from its intensity. The rainstorm center is well corresponded with the location of the strong vapor convergence. The rainstorm event is closely related to the slow movement of the typhoon lowpressure system and the sustained interaction with southwest monsoon.