An extremely heavy rainfall event (435 mm/7 h) that happened in Guolou Town behind the leeward slope of Taishan Mountain on September 2, 2005 is simulated and analyzed by using WRF mesoscale numerical model data and the Jinan SA radar data, and sensitivity tests are carried out to study the influence of the terrain of Taishan Mountain in central Shandong Province on the dynamic process of atmosphere in mid low troposphere. The results show that because of the effect of Taishan Mountain, the northeast airflow becomes circum mountain airflow in the horizontal direction, and is lifted in the vertical direction. The quasi permanent convergence zone of the circum mountain flow and gravity wave disturbance zone are formed near the torrential rain area in the lee side of Taishan Mountain and triggers the severe convection together with the convergence zone of weather system, driving the severe convective system into this area intensify and maintain. If the terrain was changed, the wind field and divergence field in low level could get weakened, affecting the location and the intensity of the extremely severe precipitation, but not having distinct impact on the distribution of the main rain belt.