Abstract: The structure of a rain-storm system and its cause of formation are analyzed in detail by high resolution numerical simulation for a rain-storm process over South China. Results show that, the meso-β rain-storm system with less than 100 km of horizontal scale occurs near the surface shear, as is the typical situation of rain-storms over coastal South China. The following features are evident with the system: strong convergence (divergence) at surface (200 hPa) and nonconvergence on mid-levels; strong upward motion exist below 200 hPa with strongest motion on mid levels; compensate sinking is found near 700 hPa; weak warm core. Three factors are found that favorable to the development of the rain-storm system: near neutro-stratification; clear energy front near the surface; strong vertical wind shear on mid and lower levels around the disturbance.