By using the data of sounding and surface dense AWS, the Doppler weather radar, the satellite cloud images and the NCEP 1°×1° reanalysis data, forming mechanism of the rainstorm on 5 June 2012 in Hexi Corridor is analyzed. The results show that this rainstorm is caused by interactions of the plateau vortex shifting northwards, the cold air moving toward southeast from eastern Xinjiang and the upper level jet. The water vapor transported to the central western corridor is originated from the northwest air flow in eastern Xinjiang and the easterly air flow of north eastern plateau. The isentropic surface over the rainstorm area is steep, atmosphere is weak and unstable easily inducing the marked increase of vertical vorticity and convergence. The dry intrusion that occurs on the south side of plateau vortex from upper troposphere enhances the baroclinity of vortex, which is conducive to vortex development and moving northwards out of the Tibetan Plateau. The rainstorm happens in the confluence area of northwest air stream from eastern Xinjiang and east air stream on the north side of the plateau vortex and in the head part of the vortex comma cloud, and the most severe rainfall occurrs near the meso β scale rebellious wind area of Doppler radar radial velocity.