A heavy rainfall and mountain torrent occurred in May 31,2005 at Taizhimiao Town,Shaoyang,Hunan Province.An analysis of the event is made based on the routine meteorological observations and hydrological rainfall,Doppler radar detection,and numerical simulation of WRF model.It shows that the important conditions causing the event are the strong instability of atmospheric stratification and severe vertical wind shear in the area.Under the background of favorable large-scale circulation,the heavy rainfall is closely related with the evolution and development of a meso-β-scale cyclone on a north-south shear line.Another condition causing the event is the formation and maintenance of a secondary meridional circulation over the heavy rainfall area.In addition,the complicated topography of Taizhimiao plays an important role in the appearance of the heavy rainfall and the outburst of the enormous mountain torrent.