By using the reanalysis data with 6 h interval and 0.75°×0.75° resolution from ECMWF, ground observation data, Micaps and TBB data, a wide range rainstorm process on 20 August, 2012 was diagnostically analyzed. Results showed that the rainstorm occurred in the favourable conditions of the coupling effects combining divergence at jet extrance at high level with the trough shear line at low level, the slow movement of the trough system and the abundant water vapor for the rainstorm belt by the indirect effects of typhoon. The main source of water vapor in the rainstorm belt was from the East China Sea and the South China Sea. Analysis on isentropic potential vorticity, moist potential vorticity and total energy vertical profile showed that in the condition of abundant water vapor, the joint effects of convective instability superimposed with baroclinic instability as well as symmetric instability caused the rainstorm. Furthermore, the south and north air flow converged strongly in the heavy rain area (upward at south side; downward at north side) also played an important role. Vertical profiles of total energy quite well corresponded with the convection strength and strong precipitation periods. The southeast warm air and the dry-cold northeast airflow at low level triggered this wide range rainstorm process.