Abstract: Using datasets of conventional observations and NCEP/NCAR reanalysis, we have quantitatively analyzed the dry intrusion effect in a torrential rain event occurred on 21 July 2012 in Beijing. The results show that the dry intrusion phenomenon was the mostly pronounced before the rainstorm occurred. The dry intrusion index can reveal the heavy rainfall area and rainfall reinforcement. The dipole distribution pattern of dry intrusion index reflects the interaction of the cold and warm air masses during the genesis and development of extratropical cyclones. Heavy rainfall always occurs in the dense and negative dry intrusion index contour zone. The closer the zero contour line is in the plus or minus centers of dry intrusion index, the stronger the rainfall shows. In the whole rainfall process, the positive value center of dry intrusion index in the mid- and upper-level extended steadily to the medium level. In the vertical direction, when the dry intrusion index changed from consistently negative value to alternate positive and negative values, especially the dry intrusion index changed from negative to positive in the middle troposphere, that is, unstable stratification with the dry and cold air in the middle, and warm and humid air in the low levels, convective precipitation is easily triggered, which makes the rainfall reinforced. The dry intrusion index represented by isentropic coordinates can similarly used to depict the dry intrusion process, and has clearer indication significance in the lower and middle isentropic surfaces than in upper isentropic surfaces. In addition, the total layer' s (200-950 hPa) mean dry intrusion index has a 6-12 h response time ahead to this torrential rain event.