Abstract: Using the meso -scale model MM5, the numerical simulation of the evolution of a meiyu front rainstorm occurred over the area around Anhui-Zhejiang-Jiangxi in China on 9 June 2008 is performed and analysed with emphasis on the generation and development of a meso β convective system (MCS) embeded in the meiyu front based on the outputs from the MM5 of high resulution. The analyses of some physical quantities such as vorticity and divergence at the center of the MCS indicate that their first rapid growths happen within the boundary layer that plays a very important role in the earlier development of the meso βconvective system. The evolution of potential equivalent temperature shows that the latent heat release is the main physical process contributed to the subsequent development of the system. To further investigate the developing mechanism of the MCS, the Ekman p umping and Ekman non balance flow were calculated from the model outputs with the result that at the early stage of the MCS, the secondary circulation was mainly driven by the adjustment of Ekman non-balance flow in the boundary layer with no effect of latent heat release and minor orography dynamic lifting as well as minor Ekman pumping contribution (of about 30%) to the ascending motion. Moreover, this secondary circulation may cause the outbreak of latent heat release and trigger the development of the MCS. Finally based on this simulation and diagnostic analysis it is concluded that in this meiyu front rainstorm case， at the early stageof MCS, the secondary circulation caused by the adjustment of the Ekman nonbalance flow in the boundary layer is responsible for the initial development of MCS, and that the sequent intensification of this secondary circulation favors the outbreak of free convective and latent heat release later so as to maintain the MCS further development.