By using the model WRF, the Backflow rainstorm process that occurred in Guangxi during 28-29 April 2010 was simulated. The formation mechanism of the backflow rainstorm was studied by diagnostic analysis. The results showed that, blocked by Wuyi Mountain and Nanling Mountains, the cold air can not directly affect the west of South China. When the cold air moves eastward into East China Sea, the west of South China is at the rear of high pressure. The isobaric line shows the trend of “southeastnorthwest” and the wind direction in lower layers turns clockwisely to southeast. The southeast air of back flow, flowing through South China Sea, is dry and cold in comparison with the southwest airflow from Bengal Bay and the crossequatorial airflow, so the airflows of different nature converge, forming the frontal surface and convergence line and providing lifting condition. This is the formation process of backflow situation. By diagnosing θe and moist potential vorticity (MPV), we find that conditional symmetric instability (CSI) in boundary layers and convective instability in lowmiddle layers are the instability mechanisms of occurrence and development of this rainstorm MCS. Wind convergence and frontogenesis of energy front in boundary layers are the main triggering conditions of the convection. Before the increase of the torrential rain, there is a process of accumulation of convective instability energy. The frontogenesis of energy front and convergence lifting in boundary layers trigger the release of instability energy, leading to the increase of the rainstorm. The negative anomaly of MPV has indication for rainstorm forecasting, and the positive center of helicity has good relationships with the increase of the rainstorm. The rainstorm happens in both sides of the boundarylayer front. The existence of boundarylayer front means this backflow rainstorm belongs to frontal precipitation in nature.