Abstract: NCEP 1 ° × 1 ° reanalysis, TBB, local density observation data and WRF forecast assimilated by Radar are used to analyse mesoscale characteristics of the abrupt rainstorm occurred at Guanzhong on 12 August 2014("0812" rainstorm). Results show that: Prediction performance of "0812" rainstorm by WRF assimilated with C band Radar is improved effectively. On the influence of cold air intrusion from north at middle and upper levels and southward cold shear at low level, meso-β gravity waves including the north to southward part around the northern Guanzhong and the east to westward part around the eastern Guanzhong are produced successively. The latter wave near the surface wind shear line. Wavelength of Mesoscale gravity fluctuation are about 60km and duration about 4 hours. Gravity wave dissipated and rainstorm stop after connection between the north part propagating southward and the east part of waves. Intensity and develop height of upward vertical velocity and rainwater content reach maximum near fluctuation region just before the rainstorm. At precipitation beginning, positive Moist Potential Vorticity (MPV) in upper troposphere over the northern part of Shaanxi Province is significantly southward and downward to Guanzhong and MPV with its vertical gradient are anomaly increase over rainstorm center. Mesoscale fluctuations of MPV are conducive to the mesoscale wave development. Vertical wind shear significantly increase and move eastward and southward around areas of south side of Jet flow near western Guanzhong at 200 hPa provide energy for mesoscale fluctuation development and propagation. Synoptic scale horizontal trough moving eastward at 500 hPa near central Mongolia and leading northerly cold air move southward at low levels, which is the direct affecting system on abrupt rainstorm. Humidity increased quickly at 850 hPa provides thermal and moisture condition for rainstorm. Significantly southeast wind near ground and unstable disturbance surge and ground shear line are favorably triggered and dynamic conditions. At the western end of ground shear line, pressure changed during 3 hours appear a feature of significant mesoscale fluctuation. Features between "0812" rainstorm and those of typical Midsummer are obviously different. "0812" rainstorm occurred under the circulation when the Western Pacific subtropical High far from the mainland of China and there are no typhoons at Chinese southeast coast. Meanwhile, the northwest vapor transportation to Guanzhong at low levels is obvious. The vertical distribution of stable stratification above 700hPa and unstable stratification near ground is beneficial to the development and propagation of gravity waves. Cloud height above rainstorm is low, meanwhile, intense precipitation echo by Radar are mainly located between the northwest and northeast wind of cold shear zone at low level.