On 21 July 2012 Beijing experienced the most severe rainfall event since August 1963. The extreme rainfall induced flooding killed over 100 people and the property damage is over 11.64 billion RMB yuan (about 2 billion U.S. dollars). Based on the routine upper level and surface observation, the satellite and radar data, a detailed analysis and investigation have been done on this event. The major results are as following: (1) The upper level trough accompanied by surface cold front moving toward east blocked by the subtropical high provides favorable synoptic scale conditions for torrential rain event in Beijing area. (2) The presence of a tropical cyclone over the South China Sea near the coastline led to the establishment and enhancement of southeastward and southward low level jet toward Beijing area, providing plenty of water vapor to Beijing area. (3) The development of Hetao vertex on 20 July led to the formation of a meso α scale MCS over that area, and its high value of vertical helicity made it well organized and longlived. This MCS moved with the upper level trough eastward, and was over Huabei region （including Beijing area） on the second day (21 July), producing extreme rainfall over Beijing area. (4) The south east low level jet constantly triggered new convective cells on the east slope of the Taihang Mountain Range, then moved to northeast direction into Beijing area, leading to the torrential rainfall and severe flooding there.