Abstract: Based on the ground，sounding conventional observation data，combined with the NCEP/NCAR global data assimilation system（1°×1°）reanalysis data，FY-2 d satellite images and the regional stations’data，a dynamical diagnosis and a mesoscale analysis of a convective rainstorm process from 20 to 21 in July 2012 in the eastern part of the Hexi Corridor，Gansu Province，China is made. The results show that: the torrential rain occurred when large-scale circulation system of high-lever troughs was cooperating with low-lever vortex，which is called as ‘north-trough and south-vortex’type，appeared. Tsaidam vortex’s eastward moving provided a favorable mesoscale circulation background for the rainstorm. This process was equipped with the characteristics of type Ⅱ cold front， strong lifting motion near the front cooperated with the convergence of low level shear line supported dynamic conditions for the rainstorm. Rainstorm occurred in the left side of south low-air jet where the south low level jet and north jet stream met，and the right side of the upper level jet stream. In the same time，the coupling of high and low jet provided profitable environment for rainstorm’s occurrence and development. Satellite imagery shows that the storm was caused by several meso-and small-scale convective clouds，which was evoked by frontal cloud band. The period of rainfall intensity’s maximum appeared in TBB’s gradient value area when it changed from small to big values. Water vapor condition reflected that during this rainstorm，apparent moist tongue and the center of water vapor convergence appeared in the low lever above Wuwei City，Gansu Province. The intense of convergence center reached -56×10^-7 g·cm^-2·hPa^-1·s^-1 on 850 hPa lever，increased five times than pre-rainstorm. Dynamic conditions showed that when rainstorm appeared，the center value of rainstorm relative helicity reached 82 m²·s^-2，and greater than the relative helicity threshold（70 m²·s^-2）. The center position and intensity of storm relative helicity was well correlated to the location，intensity and starting and ending time of this intense fall. Stability conditions indicated that stability parameters presented obvious change before and after the rainstorm. During this rainstorm，Σ Tσ，K index’s maximum value reached 406 ℃，37 ℃，more than the threshold value of local heavy rains，this indicated that there is a big potential instability energy in the process of the rainstorm. CAPE value reached 956.8 J·kg^-1 and the SI value reached 2.6 ℃，indicated that heavy rain appeared in the strong unstable stratification. This condition is conducive to the occurrence of strong convective weather. Chart V-3θ indicates that extra low temperature existed before and during this rain storm，which was beneficial to the generation of this rainstorm，and a clockwise tumble flow appeared on the middle and low levers，which is conducive to start a convection lifting mechanism.

Key words: the Hexi corridor, convective storm, diagnostic analysis, convective instability