Abstract: By employing the NCEP/NCAR reanalysis datasets, the characteristic of the torrential rain event occurred in Shandong Province is analyzed. The results show that the interaction between mid and low latitude systems play an important role in the occurrence of the torrential rain event. The feature of dry air intrusion is also analyzed, it is different from the “7.18” torrential rain occurred in Jinan and mainly presents a zonal intrusion characteristic. Through the diagnostic analysis of pseudo-equivalent potential temperature（θse）, it is found that the dry air intrusion is advantageous to the formation and maintenance of frontal zone, in addition, the dry air intrusion can cause the increase of convective instability which is useful to the maintenance and development of convection. The analysis of MPV (Moist Potential Vorticity) shows that the probably mechanism of the dry intrusion is that the high potential vorticity descends from the upper troposphere to the lower level, therefore, the dry air intrusion is one kind of the high MPV forcing.