MICAPS data, Doppler radar data, NCEP data, etc. are used to analyze the characteristics and causes of a meso β scale flood causing rainstorm on Loess Plateau from 20:00 BT 26 to 20:00 BT 27 July 2012. The results show that the activity of low energy ratio tongue in the surface layer is one of the trigger mechanisms for the two severe raifalls. The two periods of torrential rains both correspond to the establishment of easterly winds relative to storm airflow faster than 20 m·s-1 that is adjacent to the north of the storm zone within 1 km of the boundary layer, while easterly winds relative to storm airflow within 1 km of the boundary layer close to the north of the storm zone get weakened and vanished, or the range drastically reduces, causing the severe precipitation to lesson or stop correspondingly. Analysis on vorticity revenue and expenditure shows that before the first rainstorm the dominant factor is horizontal advection item in the formation of strong negative vorticity revenue and expenditure near 300 hPa. Before the second rainstorm, the dominant factors are twisting item and vertical transportation item in the formation of strong positive vorticity revenue and expenditure in the 450-300 hPa at the same time. Analysis on apparent heat source and apparent moisture sink shows that before the first rainstorm, the appearance of apparent heat source near 300 hPa is quite strong cooling layer. Before the second rainstorm, positive heating layer is formed for the apparent moisture sink is much larger than the apparent heat source in the 800-500 hPa. The formation of strong heating layer whose peak is close to 11 K·(6 h)-1 for the heat source is much larger than apparent moisture sink in the 500-200 hPa. Due to the role of the apparent heat source, cooling layer is formed in 200-130 hPa.