Based on TBB data, Doppler radar, AWS data, NCEP data and conventional observation data, the mesoscale convective system which caused the short time torrential rain in Nanjing on July 7, 2009 is studied. The results show that under the favorable weather background situation, multiple mesoscale convective systems mergered in Nanjing, and produced a new mesoscale convection system, which has strong convection and moving slowly, thus caused a short time torrential rain. The short time torrential rain’s mesoscale characteristics reflected in TBB data are that the convective clouds after mergering intensified and low temperature region obviously increased, moving slowly. The big value area of TBB gradient stayed in Nanjing for a long time. The mesoscale characteristics reflected in the surface wind field are that the combineation of the southward ground wind convergence line and the convergence center locally generated in Nanjing caused the surface convergence obviously increased. The mesoscale characteristics reflected in radar data are that the convergence in Nanjing obviously increased which is caused by the convective cell continuously mergered, high precipitation efficiency of low centroid muti cell strom and the misoscale cells passing by Nanjing along the same direction. Additionally, the shear line and the low level jet stream are the impotant influence system of the he short time torrential rain. Using Doppler radar may identify and analyze this development, and lay the basis of the short time torrential rain’s nowcasting.