According to diagnostic precipitation methods of calculating precipitable water, pseudo convection precipitable water, flux and flux divergence of moisture, cloud liquid water and cloud ice crystal, moisture weighted mean wind and condensation function precipitation rate, the continuous torrential rain that occurred over South China in June 2010 was diagnosed for its large scale moisture situation. Meanwhile, under the synoptic system of “upper trough-southwest vortex (SW vortex)-shear line-low level jet”, the extremely severe rainfall seen in Jiangxi, Fujian and Hunan in 19-20 June 2010 was simulated by a three nested, WRF model. The simulation and diagnostic analysis show that the large scale moisture field of the heavy rains in South China is characterized by “positive anomalous in the middle high latitudes-negative anomalous in the middle low latitudes-the subtropical ridge being positive anomaly towards west-anomalous activities of the monsoon trough” in East Asia, which causes the large scale vapor over the Bay of Bengal and the South China Sea to move into the Mainland of China. Many mesoscale rainstorms develop in high value areas of saturated water vapor, cloud water and cloud ice, where wind convergence and ascent motions take place and there are warm humid jet streams in its upstream. Both precipitation rates of condensation function and flux divergence of total water are products of the wind divergence initiating vertical motions, and the precipitation rate of condensation function gradually approaches approximation reasonable for explicit precipitation physical process in model atmosphere along with the improvement of spatial and temporal resolutions. When the model resolution reaches 4 km, rainfalls with the condensation function precipitation rate being 1-3 mm·min-1 can be simulated in the system of “SW vortex-shear line”, and the precipitation rate of collection growth of liquid and/or ice droplets in super saturated clouds can be described by their flux divergences and should be added to the precipitation rate of condensation function. Therefore, synoptic dynamic factors for the mesoscale rainfalls to occcur and develop in the model atmosphere is only the wind divergence field in weather system, for the vertical motions are determined by the divergence field with high spatial and temporal resolution in model atmosphere, so are all of the precipitation rates.