Based on the FNL 1°×1° reanalysis data，the causes of the rainstorm process occurred on 3 and 4 July 2012 in Qinba Mountain in the southern Shaanxi Province are analyzed, and the rainstorm is simulated by using the mesoscale model WRFV331, focused on the influence of topography on this rainstorm process. It is showed that the main affecting systems include a plateau trough at 500 hPa, a vortex shear line at 700 and 850 hPa, the southwest airflow along the northwest side of Subtropical High. The southwest jet from Yungui Plateau to the southern Shaanxi Province at 700 hPa climbed in the southern region of Qinling Mountains, and converged with the easterly wind, which maintained and developed the low vortex with shear line. The vertical motion caused by the superposition of convergence and the climbing increased precipitation in the south slope of Qinling Mountains. The orographic effect of Qinba Mountain increased the precipitation in the mountain area. The cold air in back of the Plateau trough entered Qinling Mountains from north to south, and warm air transported from the southern Qinling Mountains formed the baroclinic front zone. The convergence and uplift of the lower atmosphere caused by topography strengthened the frontogenesis and the baroclinic instability. The analysis of cloud hydrometeors indicates that during heavy rainfall, the maximum value centers of cloud water, rain, and ice in vertical direction overlapped, in favor of the seedingsupply effect, which increased rain particles and precipitation in mountainous regions.