Using conventional observations, automatic meteorological observation data, NCEP 1°×1° reanalysis data and the data from the satellite and Doppler weather radar, the torrential rainfall event that occurred over South China from 19 to 20 May 2015 was analyzed regarding the synoptic conditions, vertical structure and mesoscale convective activities. The results show that the three rainfall centers during this rainfall process were caused by three different mesoscale convective systems respectively. Firstly, the heavy rainfall over the north of Guangxi was caused by the mesoαscale vortex and shear line in the low level with the obvious frontogenesis and intense uplift. The mesoscale linear convection located behind the cold front lined more intensely, causing the rainfall to maintain a longer time with more total precipitation. The global model shows a good prediction. Secondly, the heavy rainfall over the central and northern Guangdong was triggered by the weak cold air in the boundary layer. The mesoscale linear convection moved southward because of the newborn cells in the warm and wet areas in the south. The linear structure was less intense and moved rapidly, so the total precipitation was less than that in the north of Guangxi, but the local rainfall was more intense. The mesoscale convergence line in the boundary layer and the mesoγscale convective systems played an important role. The mesoscale model can reflect the mesoscale process to some extent, but has more difficulties to forecast it. Thirdly, heavy rainfall over the southeast of Guangdong in the warm sector was triggered by the land and sea topographic uplift, persisting for a long time due to the combination with the MCSB later. The forecast capability of the model is limited. Therefore, the numerical model has different forecasting capabilities according to the different characteristics of the torrential rainfall. Understanding of the characteristics of the heavy rainfall, some corrections can be achieved by subjective forecasting.