Abstract: Characteristics of mesoscale system and dynamic factor index during disastrous short-time rainstorms that occurred at Xi'an on 3 August 2015 (The"0803"process) and 24 July 2016 (The"0724" process) are analyzed by using NCEP 1 °×1 ° reanalysis data, ground intensive observation data, satellite cloud data and WRF based on C-band radars data assimilation. Results show that Guanzhong is located in the stable northwest border of the Pacific Subtropical High. Westerly trough and shear line are the direct systems that influence. In the afternoon, upper dry and lower wet stratification with air at lower levels is hyper adiabatic, which provides favorable environment for convection and strong release of unstable energy. Accompanied by the passage of a cold front, jumping of northward wind at surface from northwest Xianyang and Tongchuan in the afternoon is the triggering to rainstorm. Rainstorm is near the 3-hour anallobaric center of the ground to the east side of the great gradient region. The disastrous short-time rainstorm clouds are located far from the northern upper Jet, and their north parts have no baroclinic and leaf-like forms. The rainfall drag effect is evident. TBB center decreases to -65℃ during the rainfall peak. Graupel is the most significant particle in the mesoscale convective cloud above middle levels. The strongest upward motion center is located near the -30℃ level and snow center, which is just above the graupel center. There exists five times increasing of different particle content in the mesoscale convective cloud when rainstorm begins. The rainfall intensity peak is consistent temporally with that of snow particle content. Environment conditions and mesoscale characteristics are obviously different between the two processes. The"0803" process is near the cold zone of shear line, which features a wide range of upper westerly jet currents, a southerly location, a weaker Western Pacific Subtropical High, an active northerly cold air behind the westerly trough and weaker southward water vapor transport. Meanwhile, there are obvious energy front and symmetric instability below the middle layers of Guanzhong. The strong precipitation is concentrated near the energy front. The ice water particles in the cloud cluster are obviously more, and the proportion of snow particles are higher than that of graupel particles. The rainstorm center lies near the maximum gradient region of TBB at the edge of the convective cloud. Cold air intrusion is a favorable condition for the rapid growth of cloud ice particles. The "0724" process is near the warm zone of shear line, which is accompanied with small range upper westerly jet currents, northerly location, stronger Western Pacific Subtropical High, weak northly cold air behind the westerly trough and stronger southward water vapor transport. Meanwhile, there is potential instability below the middle levels near Guanzhong. The ice water particles in the cloud cluster are fewer, and the ratio of snow particles to graupel particles is low. The rainstorm center is located near the TBB center which is inside the convective cloud. Vertical Helicity Index (VHI) and Divergence Vertical Flux Index (DVFI) have good reference value for the forecast of short-term rainstorms in Xi'an. The rainstorm is located near the area where the centers of anomalous large values of the dynamic index tend to be concentrated and stable. No heavy rain happened near regions with dynamic index less than 200, or with scattered centers and short duration.