Abstract: A rainstorm belt of about 80 km and 350 km in the EW and NS direction respectively was generated by a cloud system in front of the upperlevel trough from 20:00 BT 6 July to 14:00 BT 7 July 2008, covering from Nanning city to Huanjiang county, Guangxi Province, China. A mesoscale convective system (MCS) in the south of the cloud system triggered a torrential rain with precipitation of 265.0 mm in Shanglin county. The FY-2C cloud images, radar observations, records obtained from automatic weather stations and other irregular observations were used to analysis this process. Based on the cloud image and isentropic analysis charts, the dry air moved down across the YunnanKweichow plateau to the west of Guangxi Province and the north of Vietnam after the upper-level trough passed the Tibet plateau, and it turned eastwards at the upperlevel trough bottom to the middle of Guangxi, and then formed a mesoscale vortex and a mesoscale katallobaric center near Shanglin county. After that the MCS developped and generated heavy rain in the mesoscale vortex and katallobaric center. The severe rainfall occurred 2 hours after the mesoscale vortex and katallobaric center occurrence. The results show that the main favorable factors generating the MCS are as follows. Firstly, the sufficient water vapor was transported by the warm and wet southerly flow ahead of the upper-level trough to the middle of Guangxi to form the unstable potential, which provided an advantageous environmental condition for the MCS generation. Secondly, the MCS was generated while the warm and wet airflow was uplifted by the convergence and the ascend motion of the mesoscale vortex. Thirdly, convective cells moved forward along the low level shear line, which made the MCS maintain and develop. Furthermore, a 2D conceptual model of the MCS development mechanism is presented.