Based on the reliable output of the MM5 model, the heavy rain occurring in the w arm sector in Guangxi during 8-9 May 2005 was analyzed. Results show that the l ow level jets (LLJ) kept on strengthening and the upper level jet maintaining ar e responsible for the dynamical mechanism; and in the lower troposphere there is the appearance of the cores of the LLJ moving eastward from the upper to the lo wer levels, which is the key reason for the heavy rain happening far away from the front and occurring in the warm sector. The organized convective movements repeatedly developing in the warm sector make the precipitation amount and rainfall rate more and stronger than those in the front area.