Based on station observations (surface observations and upper-air soundings at weather stations and precipitation observations at regional stations), FY-2D infrared images, water vapor images and NCEP reanalysis data with the resolution of 1°×1°, combined with water vapor flux diagnosis and a backward trajectory model, an extreme rainstorm process which occurred in the arid region of western Hexi Corridor during 15-16 June 2011 was analyzed to study the transport mechanism of water vapor and its budget in the process. The results are as follows. The western Hexi Corridor was affected by the plateau vortex center formed from the western part of warm high-pressure ridge which ran northwest-southeast from western Inner Mongolia to Hetao Area. The plateau vortex center with 200-400 km scale remained stable over the rainstorm area for more than 12 hours, forming good dynamic conditions. The humidity increased and the temperature decreased in the lower troposphere. The cold air invaded from the southern plateau vortex in middle layer. The atmosphere was in a weakly unstable state. The convergence of surface wind speed and the effect of terrain upwind slope triggered local convective weather. The water vapor needed by rainstorm was mainly from the west wind flow and the east wind flow bypassed the plateau, and the latter brought the most water vapor with its contribution rate during rainstorm as high as 84.6%. Further more, the water vapor from two channels were both the most pronounced in the middle and low layer of the tro-posphere. The formation of anomalous easterly airflow in central Gansu at 500 hPa and 700 hPa was important for water vapor transport and convergence in the rainstorm area. The average net input intensity of water vapor during the rainstorm period was 2.73 times of that the pre-rainstorm period. The low vortex center in the middle troposphere, the convergence of wind speed and direction in the lower troposphere, and the convergence of terrain caused the 550 hPa, 700 hPa, and 800 hPa three-layer water vapor convergence center with 200-400 km scale. The atmospheric precipitable water in the rainstorm center was as high as 34 mm, which was more than 2 times the summer average.