Using the Weather Research and Forecasting (WRF) model, numerical experiments are carried out to simulate a sudden severe rain in southwestern Guizhou Province on 12 Jun 2006. The results show that this severe rain is caused by locally rapid-developing meso-β-scale convective systems (M-βCSs) . At the beginning, the initial ascending movement was driven by the mesoscale shear line formed in the lower layer of atmosphere. M-βCSs were developing in the instable warm air in front of the surface cold front, the south wind transporting moist warm air on the south of the convergence zone was the most important factor for sustaining the development of these convective systems. Through diagnosis on the basis of ageostrophic omega-equation, it is showed that the release of condensation heating has an important effect on the development of M-βCSs, the ascending movement forced by it was greater than the contribution of the low-level warm advection. Through the thermodynamic and dynamic adjustment, some dynamic structures such as mesoscale Low Level Jet (mLLJ) and mesoscale vortex appear in the lower troposphere. Near the ending of the rain, the invading of the north flow made the lower layer of atmosphere convert into stable state from convective unstable state, so there was not enough instable energy for maintaining vertical motion, and the precipitation stopped in the end.