Abstract: The landing of a strong tropical storm named Bilis (200604) created the favourable conditions for the formation of a longlasting lowpressure system, whose interaction with the South China Sea (SCS) monsoon led to a peculiar rainfall in the southeastern part of Hunan Province. Based on various conventional observations, unconventional finegrid observations, the NCEP reanalysis data, and the data from the mesoscale Advanced Regional Etacoordinate Model (AREM), this study performed numerical simulation and diagnostic analysis of the characteristics of moisture transport during the heavy torrential rainfall process. A sensitive experiment was designed to reveal the specific water vapor source and the flow channel for the torrential rainfall in the southeastern part of Hunan. The results indicate that the interaction of Storm Bilis (2006) with the SCS monsoon played a key role in the moisture transportation during the heavy rainfall event. East of the storm center, there maintained a band of strong southerlies that carried moist air from the SCS monsoon to the north of the storm, where the moisture was continuously transported to southeastern Hunan by the increased northeast winds. The intercrossing of these northern and southern moisture currents formed a deep moist layer and strong vapor convergence over the southeastern Human. It played the most important role in maintaining Storm Bilis for a long period and producing the heavy rainfall over the southeastern part of Human.