There was a torrential rain occurring in Beijing during 15:00—18：00 BT 10 August 2008, and it was the afternoon of that day there were three mesoβ convective cells that moved to city of Beijing, generated heavy rainfall (can be 45 mm/h and 12 mm/5 min) and had an obvious organizational development. Many high spatialtemporal resolution data, such as autoweather station (AWS) network data, Doppler radar data, wind profile data and variational Doppler radar analysis system (VDRAS), are used to analyze the organizational development of the mesoscale convective system in this case. It is revealed that: (1) the cause for the organizational development of three convective cells in the afternoon is about the appearance of mesoβ shear which was in the front of stratified precipitation system; (2) there are obvious changes before starting to rain in main urban areas of Beijing, such as accumulation of CAPE, and environmental wind whose direction changed from south to east; (3) the casue for the origanizational development of the three convective cells is due to the shear generated by the confliction between the northwest cold horizontal flow from stratified rain system and the southeast warm horizontal flow beneath 500 m. Therefore, at the advantaged largescale background, with regard to the weather forecast of largescale rain event, one of the key places is in the front of stratified precipitation system, where conflict between the cold horizontal flow and the environmental wind before the system maybe triggers the organizational development of convective system.