Based on the routine data, NCEP data, satellite images, radar data, and the dens ified observations, a dynamical diagnosis of the convect ion torrential rain in south of cold front on 28 May 2008 in the northern part o f Jingxi Province is ma de. The results show that (1) the convection torrential rain south of cold front was due to the several main reasons, such as, all nearly vertical troughs at various levels, precipitous front surfaces, forwardedtilting trou gh structure, extremely strong lower warm and wet air flows ahead of the cold front, greatly increased unstabl e energy of the convection formed by warm, wet below and dry, cold above convect ions, and the lowlevel southwest jet stream strengthening when the cold front moving closely and cyclone wave moving east. All of these factors resulted in t he unstable energy release from the convection. (2) This torrential rain showed the characteristics on obvious meso and small scales that had four mesoβ scale convection systems moving to the east along the ground cold front. The gr ound mesoscale cy clone, the mesoscale convergence line, the strong echo area, the head wind zon e, and the 850hPa mesoscale convergence line, all demonstrated the presence of the mesoscale perturbation. Furthermore, mesoscale perturbation was well corre lated to the convective rain cluster.