Abstract: Extreme rainfall that occurred in Beijing on July 21, 2012, was the most severe rain event of a 61-year history in that region and caused significant damages. National Centers for Environmental Prediction (NCEP) reanalysis data shows that interaction among Typhoon Vicente, the subtropical high (SH), and the southwesterly monsoonal flow (SMF) played an important role in this rainfall event. In a process known as the moisture alteration effect of a typhoon, Vicente received moisture from the SMF, which was transported to the Beijing region through the southeasterly flow. To verify this effect, the rainfall event is simulated in this study by using the Weather Research and Forecast (WRF) model with three nested domains and a finest resolution of 3.33 km. The control experiments effectively reproduce the distribution and intensity of the rainfall and moisture transport. The results of a sensitivity experiment excluding typhoon Vicente, which is conducted to determine its impact on the extreme rainfall, reveals that approximately 50% of the rainfall can be attributed to the storm. Further analyses of column moisture flux from the sensitivity experiment suggest that more moisture was transported to Beijing through southwest flow that from the southeast. This transport from the southwest to typhoon Vicente and then to Beijing disappeared, which corroborates the moisture alteration effect of the typhoon.