Based on slantwise vorticity development(SVD) theory,the possible dynamical mechanism of a persistent torrential rain which occurred in mid-June of 2008 was analyzed,and the role of low level jet (LLJ) in vorticity development was discussed.Meanwhile,several numerical simulations were performed to testify the rainstorm process based on MM5 and the results were further analyzed.The results indicate that:the development of low vortex from Tibetan Plateau was the primary cause for the precipitation in Guangxi and Guangdong at early stage.SVD took effect while moist isentropic surface tended to slope and air particles had relative motions along isentropic surface.With certain condition (C_M<0),the vorticity got a further development afterwards.Moreover,the enhancement of LLJ changed the atmospheric baroclinity and caused the vorticity to develop and move eastward,which was the main cause for the persistent torrential rain in Guangdong.The results of simulation control test well revealed the development process of the vortex and precipitation.The result of sensitive test showed that LLJ changed the stability of low level atmosphere and vertical shear of the wind,which meant that the baroclinity of the atmosphere was increasing.It thus made the mesoscale vortex develop further.