Using the GRAPES_Meso model, the numerical simulation about the persistent flood rainstorm is carried out in Shaanxi and Sichuan Provinces from 16 to 18 September 2011. By comparing the three sets of sensitivity test results, it is revealed that the interaction between the airflow and the terrain contributes to the disaster rainstorm, and the detailed results are as follows: (1) by the common influence of typhoon and Mongolia high pressure during 16 to 18 September, an eastern water vapor channel is formed over the Yellow Sea and the Bohai Sea at 850 hPa, making a large quantity of moisture through the North China Plain transported to the West China region. The vapor is divided into two parts by Qinling Mountain: the north one and the south one. The north vapor is blocked and uplifted by Qinling Mountain and forms a water vapor convergence center. The south one enters the Hanzhong Plain and is influenced by the “bell mouth” terrain, which results in convergence and upward. (2) The parallel alternate ridges and valleys create positive and negative vertical wind band, forming a strong suction effect in the vertical direction, and moreover a plenty of water vapor convergence occurs in the same area, triggering the large scale severe precipitation. (3) The altitude and the sufface relief have great influence on rainfalls, and the greater the surface relief is when the altitude is the same, the easier rainstorm occurs.