Abstract: Based on NCEP reanalysis data and WRF model，circulation feature analysis and numerical simulations are carried on for the extreme torrential rain in Yunyang on 31August，2014．In allusion to Daba mountains and Qiyue mountains，terrain sensitivity testsare designed．It is indicated that southwest vortex，shear line and low-level jet are the direct effect systems of this progress．The function of Daba mountains is obstruction and wind slope effect．With the reduction of the height of Daba mountains，low vortex move obviously northwards，precipitation cell move northeastwards，the weakening of low-level convergence brings about the weakening of intensity and ceiling of convection of the ascend branch of secondary verticalcirculation，which contributes to the decrease of the precipitation．The function of Qiyue mountains is funnelling．With the reduction of the height of Qiyue mountains，canalization minnished，low-level jet get weakend and move southwards，the position of low-level convergence move further south，which lead to the southward deflection of the ascend branch of secondary vertical circulation．