Abstract: By use of NCEP1°×1° data for reanalysis and data for floor precipitation, an analysis is made of the rainstorm process that took place in Sichuan and Chongqing between August 16 and 19, 2015. The results showed that the eastward movement of the conditioning tank in west Mongolia and the westward extension of the subtropical anti-cyclone of the western Pacific Ocean offer a favorable background condition for the formation of a low rainstorm vortex; the slow eastward approach of the southwest vortex, which originally formed in the south of Sichuan, to the northeast of the province, reinforced in the process of mergence between the high and low altitude vortex and the sufficient water-vapor transfer in the bottom layer are the key elements to bring about the rainstorm.