In order to understand the rainstorm process in warm section over southern China, the rainstorm occurring on May 9, 2005 is analyzed using the synopticobservation data and satellite image firstly. Then the numerical simulation of the rainfall is conducted by using MM5V3.6 model. Based on the outputs of successful simulation, the structure and cause of the mesoscale rainstorm system are studied. Results show that the divergencegenerated on the right side of the upper level jet entrance region overlapping the convergencegenerated on the left side of lower level jet exit region in southern part of South China leads to the heavy rain happeningin the warm section. The mesoscale convective clouds are direct affecting system of the rainstorm. The southerly flow vertically ascends strongly over the rainstorm system and the ascending draft forms a meridional verticalcounter-circulation with the downdraft in upper troposphereon the southern side of the system in cross-section along longitude. The velocity convergenceof lower level flow plays a crucial role in formation and development of the heavy rain system of warm sector. It is indicated that the higher rotational helicity might be one important mechanism for the rainstormand can be used to judge the development and movement of the rainstorm system.