The atmospheric circulation, sources of water vapor, mesoscale systems related to extremely heavy rainfall over the Sichuan Basin on 8 July 2013 are analyzed by using the conventional data, surface dense observation, satellite data and NECP reanalysis data. This process occurs in the background with “high pressure to the east and low pressure to the west”. A small trough moves eastward over the eastern part of plateau under the stable weather pattern of westerly trough and subtropical high over Sichuan Basin. The southwest flow from the Bay of Bengal veers to easterly in Sichuan Basin transporting the moisture to western Sichuan Basin. The surface mesoscale convergence line which developed from surface easterly and northerly winds is the favorite condition for the development and strengths of convective cloud cluster. The simulated results from WRF model show that the effect of steep terrain in western Sichuan Basin and the development of low vortex provides favorite conditions for development of the convective system. The north wind invading to the north of the surface convergence line induces to the realease of convective energy, which is also favorite for the outbreak and maintainence of convective systems.