Based on the conventional observations，NCEP / NCAR reanalysis data，FY － 2E satellite images and radar data，the synoptic analysis and physical field diagnostic analysis methods were used to explore the triggering mechanisms and vapor sources of the heavy rainstorm that occurred in Northwest China． By analyzing circulation patterns，vapor，dynamic and unstable conditions，the results show that the southwestern low level jet and the eastern low level jet played significant roles during the heavy rainfall process，they brought abundant water vapor and unstable capability． Forward trough and warming humidification in the low layer made stratification convective instability．The overlay of convergence line on 700 hPa and the shear line on 850 hPa was the triggering condition，which resulted in the release of unstable capability and induced the heavy rainstorm． Meanwhile，convergence at low level and divergence at upper － level formed upward motion over the entire layer，which was the dynamic condition．The study not only provides focus for forecasting heavy rainstorms in arid Northwest China，but also supplies references to control flood and mitigate disasters for local government．