During 12-13 June 2008, a heavy rainfall event mainly caused by the successive mesoscale convective systems (MCSs), which experienced genesis, merging and intense development, occurred in southern China. By using a new generation mesoscale model of Weather Research and Forecasting (WRF), this heavy rainfall event is simulated, and the mechanisms of MCSs genesis, development and evolution are also analyzed. During the genesis of MCSs, they were located in the environment with positive vorticity due to the presence of the southwest vortex. Both of the convergence at lower levels induced by positive vorticity and static instability are important mechanisms for convective initiation. The collective effects from them are favorable to the convective initiation and enhancement. After generation, MCSs can influence the surrounding vorticity by tilting in the strong vertical shear environment, and then the change of surrounding vorticity will affect the MCSs. This positive feedback causes the rapid development of MCSs. In the process of interactions between these strengthened MCSs and environmental flow, the merging of MCSs is completed. In addition, the Magnus force acts on the splitting of MCSs.