A heavy rainfall (>330 mm/12 hours) event occurred in Shanghai City on 5-6 August, 2001. This torrential rainfall made many streets in Shanghai submerged and caused a lot of damage due to the lack of city drainage capability. This study present the mesoscale process of torrential rainfall using the data from a mesoscale observed network and the result of a numerical simulation of this heavy rainstorms event using the Penn-NCAR Mesoscale Model version 5 (MM5) with a horizontal resolution of 15 km. In order to indicate physics of this event, some numerical experiments have been done. The analysis of observed rainfall data from a high-resolution observation system in Shanghai indicates that the distribution of precipitation have a meso-g (the range of heavy rainfall is less 50 kilometers) and the rainfall of process is organized by several meso-g system. They formed in the suburb of Shanghai and moved toward Shanghai city and developed in the city area. The synoptic analysis suggests that the heavy rain event is occurred in the inner of a landfall tropical depression. This depression change its track suddenly, move back to the coast of East China Sea and stay over the Shanghai area for a long time after it landfall at coast of Zhejiang Province and move towards the Hefei, the capital of Anhui Province of the central of China. This tropical depression supplies a favorable atmospheric background for the heavy rainfall event generating. The reason why the tropical depression changes dramatically also is included in this study. The numerical simulation result suggests that the MM5 could re-produce the process of the heavy rainfall event. The results of numerical experiments indicate that the development of this torrential rain is related to the role of the disturbance of topography located in the southwest named Tianmu Mountain under the situation of tropical depression.