By employing the NCEP/NCAR reanalysis data, the authors have analyzed the detailed characteristics of the Ji'nan torrential rain occurring on 18 July 2007, the results show both the upper-level trough and the low-level wind shear are helpful to the occurrence of the torrential rain event. The most severe dry air intrusion around Ji'nan occurs 1-2 days ahead of the torrential rain event, the dry air intrusion is advantageous to the formation and maintenance of the thick dry level. The thick dry level can lead to the generation, accumulation, and release of the Convective Available Potential Energy (CAPE), and then, it can cause the convective upward air motion which can provide an advantageous condition to the occurrence of the torrential rain. On the north of Ji'nan, the dry air intrusion phenomenon can be always observed during the torrential rain event, it has two modes: in the lower troposphere, the dry air from the mid-high latitudes can arrive in Ji'nan; in the upper troposphere, the dry air from the upper levels can be extended into the lower troposphere. The dry air intrusion beyond Ji'nan can cause the divergence in the lower troposphere and convergence in the upper troposphere, it can also influence the formation and moving of the front, and both the two aspects show that the dry air intrusion play an important role in the torrential rain event.