Abstract: A rainstorm happening in South China during 8-9 June 1998 is selected in this paper. Diagnoses and numeric simulation are made on this rainstorm using observed data and MM5 model, to get the mesoscale structure of the rainstorm and the influence of complex topography. The results show that the rainstorm is made up by a frontal precipitation and a warm sector precipitation that have different mesoscale structures. The rainstorm happening at Wuzhou district in Guangxi Region is inspired by an MCS on a quasi-stationary front, which maintains a long time and has large intensity. The rainstorm happening at the Delta of Zhujiang River is a warm sector precipitaton, which also lasts for a long time and also has large intensity. The influence of the complex topography is dynamic. In the Wuzhou rainstorm, three branches of air current that are all affected by the topography converge at the center of the rainstorm. In the Zhujiang rainstorm, the warm air current is forced to move around the Hainan Island and strengthened the convergence over the Delta of Zhujiang River.