An extreme rainstorm named as ‘7·27' occurred in the middle of Inner Mongolia in China during late July 2012. Using data from NCEP reanalysis, conventional observations, and monitoring, the causes of the ‘7·27' extreme heavy rain were analyzed and discussed. The results showing: (1) Bell-Lake vortex happened in ‘7·27' rainstorm was quite different from that in perennial years. The area from Balkhash Lake to the Ochotsk Sea was controlled by blocking high, the bottom of the blocking high was occupied by Bell-Lake vortex.(2)‘7·27' rainstorm has strongly the characteristics of the meso-scale. Many meso-scale rain belts were generated by surface shear lines formed constantly and swung north and south. The β meso-scale convective clouds were produced constantly and enhanced by amalgamation and further formed the convective complexes (MCS or MCC). The rainstorms in the northwestern and southern Hetao area, and the western Xilin Gol League were all resulted from the meso-scale convective complex (MCC) or MCS and ground shear lines, (3) The distribution characteristics of the water-vapor were: the atmospheric specific humidity in vertical was up to 10~22 g·kg^-1 from ground to 700 hPa. The water vapor transported strongest in the scope from 850 hPa to 700hPa. Horizontally, the south winds reached 45°N, which made the "moisture sink" of Southern Hetao. (4) Atmospheric vortex in Troposphere was the direct cause of the event. The atmosphere vortex spread from 200 hPa down to 700 hPa. Heavy rains were hence produced by the upper atmosphere divergence and the lower compensate convergence. (5) The thermodynamical mechanics of ‘7·27' rainstorm is: The instable energy of atmosphere was rebuilt constantly with the mid-level dry intrusion and the low-level humid transport. (6) With global warming, the Asian summer monsoon intensifying, from the bay of Bengal and the South China Sea to the East Asian subtropical region water vapor transport strengthening, water vapor transport further northward expansion into North China's inland areas, is an important reason for ‘7·27' rainstorm process.